RJS-2636: Add progress notifications and tests

[gagik]

What, How & Why?

Adds progress notifications for flexible sync and tests for it.

This closes https://github.com/realm/realm-js/issues/6256

☑️ ToDos

• [X] 📝 Changelog entry
• [X] 📝 Compatibility label is updated or copied from previous entry
• [X] 📝 Update COMPATIBILITY.md
• [X] 🚦 Tests
• [ ] ~📦 Updated internal package version in consuming package.jsons (if updating internal packages)~
• [ ] ~📱 Check the React Native/other sample apps work if necessary~
• [ ] ~💥 Breaking label has been applied or is not necessary~

[gagik]

I am still battling some CI issues but I figure it may be worth requesting some reviews for now.

[gagik]

Okay, I am 99% certain that last failing tests is a result of GHA caching, and I tried to delete the cache manually many times for this branch/PR but seems to keep using it, @kraenhansen may have a better idea of what is going on here... but will have to leave it at that since this now 62 commits deep into making it fully green 😄 At least it does not seem flaky and is somewhat reasonable in its underlying implementation.

Will leave it at this point since there's no chance to have a proper reviews but feel free to continue from here to get it merged. I'd say it is pretty complete and although tests are far from ideal, there is also no clear behavior expectations yet and much of it is just up to core.

Should be good to merge beyond any review comments you can have. Feel free to get in touch if needed in next few days.

[gagik]

Just tried to reply to some of the comments that may have been hard to guess without my input. Beyond that, going to be away so feel free to go ahead with what seems best 😄

[elle-j]

Note on Android tests: Still experiencing signature mismatch.

(Reference)

[kraenhansen]

@elle-j with https://github.com/realm/realm-js/pull/6769 merged, I'd expect the Android tests to run if this branch is rebased.