feat(page): add omitBackground option for page.pdf method

[d-rowe]

Adds omitBackground option for Page.pdf to allow for transparent backgrounds. This option is available in the screenshot api, this change unifies this for both .screenshot and .pdf.

Related issues: #6906 #2545

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[d-rowe]

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[jschfflr]

Hi d-rowe, thanks for your contribution!
Your changes look good to me but it would be great if you could add a test for them.
I'm not sure how stable the pdf output is so maybe you can try testing it with golden snapshots (take a look at the screenshot tests to see how) and if that doesn't work we can find another way to at least have basic test coverage.
Thanks!

[d-rowe]

Thanks. Looks like the tests are only setup to compare pngs. Any suggestions for alternative ways to get some basic test coverage?

[jschfflr]

Well if that's not currently set up to handle PDFs, I think we can skip it for now.
But maybe you can create a tracking bug for this on this repository so that we can take a look at it in the future.
Please also link to this pull request from that new issue. Thanks!