Expose `vsync` information to developer

[fzyzcjy]

This PR tries to expose vsync information to the developer, so they can know when it is proper to call the more un-restricted window.render proposed in #36438.

Currently only the API is shown, because IMHO the implementation details is unrelated to thoughts about #36438, and the API (and therefore implementations) are subject to changes. I will continue working on it, once the API is approved.

For detailed design about this API and its implementation, please have a look at "Get last vsync time information" section of https://docs.google.com/document/d/1FuNcBvAPghUyjeqQCOYxSt6lGDAQ1YxsNlOvrUx0Gko/edit.

Sample usage:

final info = SchedulerBinding.instance.lastVsyncInfo();

List of work:

• [x] code the (draft) Dart API
• [ ] discuss whether the API is acceptable
• [ ] implement the C++ part on SDK<=29 Android
• [ ] implement the C++ part on new android, ios, and other platforms
• [ ] create a wrapper function in flutter/flutter repo, probably in SchedulerBinding

List which issues are fixed by this PR. You must list at least one issue.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the CLA.
• [ ] All existing and new tests are passing.

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