Implement selection panel refactor

[nfmohit]

Summary

Addresses issue:

• Refactor the Key Metrics Selection Panel, extracting components for reuse in the Audience Segmentation Selection Panel. #8652

Relevant technical choices

This PR refactors the Key Metrics selection panel to be a set of standalone components that can be re-used to create additional selection components.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

Build files for f6ea234 have been deleted.

[github-actions]

Size Change: +2.8 kB (+0.2%)

Total Size: 1.44 MB

Filename	Size	Change
./dist/assets/css/googlesitekit-admin-css-********************.min.css	53.7 kB	-27 B (-0.05%)
./dist/assets/js/34-********************.js	3.12 kB	+1 B (+0.03%)
./dist/assets/js/googlesitekit-activation-********************.js	23.9 kB	+41 B (+0.17%)
./dist/assets/js/googlesitekit-ad-blocking-recovery-********************.js	57.4 kB	+62 B (+0.11%)
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./dist/assets/js/googlesitekit-api-********************.js	10.2 kB	+43 B (+0.43%)
./dist/assets/js/googlesitekit-components-gm2-********************.js	5.87 kB	-6 B (-0.1%)
./dist/assets/js/googlesitekit-components-gm3-********************.js	10 kB	+2 B (+0.02%)
./dist/assets/js/googlesitekit-data-********************.js	2.18 kB	+4 B (+0.18%)
./dist/assets/js/googlesitekit-datastore-forms-********************.js	9.13 kB	+36 B (+0.4%)
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./dist/assets/js/googlesitekit-main-dashboard-********************.js	96.7 kB	+1.01 kB (+1.06%)
./dist/assets/js/googlesitekit-modules-ads-********************.js	25.4 kB	+1.19 kB (+4.9%)	🔍
./dist/assets/js/googlesitekit-modules-adsense-********************.js	110 kB	-389 B (-0.35%)
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./dist/assets/js/googlesitekit-modules-********************.js	21.7 kB	+34 B (+0.16%)
./dist/assets/js/googlesitekit-modules-pagespeed-insights-********************.js	22.4 kB	+59 B (+0.26%)
./dist/assets/js/googlesitekit-modules-search-console-********************.js	58.1 kB	-26 B (-0.04%)
./dist/assets/js/googlesitekit-modules-tagmanager-********************.js	30.1 kB	+20 B (+0.07%)
./dist/assets/js/googlesitekit-settings-********************.js	59.7 kB	-6 B (-0.01%)
./dist/assets/js/googlesitekit-splash-********************.js	71.8 kB	+54 B (+0.08%)
./dist/assets/js/googlesitekit-user-input-********************.js	46.9 kB	+55 B (+0.12%)
./dist/assets/js/googlesitekit-vendor-********************.js	317 kB	-35 B (-0.01%)
./dist/assets/js/googlesitekit-widgets-********************.js	33.4 kB	+29 B (+0.09%)
./dist/assets/js/googlesitekit-wp-dashboard-********************.js	59.8 kB	-320 B (-0.53%)
./dist/assets/js/optin-monster-********************.js	635 B	+1 B (+0.16%)
./dist/assets/js/runtime-********************.js	1.3 kB	-1 B (-0.08%)
./dist/assets/js/mailchimp-********************.js	593 B	+593 B (new file)	🆕
./dist/assets/js/popup-maker-********************.js	595 B	+595 B (new file)	🆕

ℹ️ View Unchanged

Filename	Size
./dist/assets/css/googlesitekit-adminbar-css-********************.min.css	11.2 kB
./dist/assets/css/googlesitekit-authorize-application-css-********************.min.css	731 B
./dist/assets/css/googlesitekit-wp-dashboard-css-********************.min.css	7.48 kB
./dist/assets/js/29-********************.js	2.8 kB
./dist/assets/js/30-********************.js	2.28 kB
./dist/assets/js/31-********************.js	3.72 kB
./dist/assets/js/32-********************.js	929 B
./dist/assets/js/33-********************.js	888 B
./dist/assets/js/analytics-advanced-tracking-********************.js	778 B
./dist/assets/js/contact-form-7-********************.js	606 B
./dist/assets/js/googlesitekit-i18n-********************.js	3.92 kB
./dist/assets/js/googlesitekit-polyfills-********************.js	379 B
./dist/assets/js/woocommerce-********************.js	613 B
./dist/assets/js/wpforms-********************.js	595 B

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[kuasha420]

@nfmohit Excellent work here. Mostly looking good. I've left a few comments for you, please have a look. Cheers!

[nfmohit]

Thank you for the kind review, @kuasha420. I've responded to all your comments. This is back with you for another pass.

[kuasha420]

Thank you both @techanvil and @nfmohit, excellent work here and not a ounce of VRT diff (except for the new stories, of course). This is perfect. 🎉