[FEATURE] add memory cache to @salesforce/pwa-kit-runtime/utils/ssr-config getConfig function

[luca-izzo]

Description

getConfig function inside @salesforce/pwa-kit-runtime/utils/ssr-config/ takes relatively long time to be executed on SSR and when called thousand of times can have a huge impact on the performances.

Steps to reproduce

On both remote and local environments:
Call getConfig function inside @salesforce/pwa-kit-runtime/utils/ssr-config/ a lot of times in your application and observe the impact on the rendering time.

Types of Changes

Added a Map to cache in memory the result of the function based on the provided buildDirectory.

Closes #1620

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @luca-izzo to sign the Salesforce Inc. Contributor License Agreement.

[bfeister]

@luca-izzo thanks for the contribution! Since you've already done the work of investigating the problem, and I'm sure you've compiled data and metrics that demonstrates the increasing latency with successive calls, I would like you to post those here as part of the PR description. Whenever I think about pull requests, I always think "Will it make sense for me looking back at this PR as a reference in 12 months from now when I've forgotten the discussion?"

The data in your findings (screenshots that show an increasingly slow response time in the dev server in terminal or "network" tab in Chrome are fine) will be important to help us validate both the problem and the solution

[luca-izzo]

Hello @bfeister,
I've measured one of our HPs where we call this function 694 times.

Sum of execution time of each call to getConfig in ms:
Local before: -750ms
Local after: ~1ms

Remote before: -550ms
Remote after: ~1ms

Chrome network tab:
Local before:

Local after:

Remote before:

Remote After:

[luca-izzo]

@bfeister ,

Note:
Another breaking change for us was to remove all "URL" objects.
We removed it from all helpers functions together with caching getConfig output and we've seen a drastic performance improvements.