fix(firestore): collection and document equality checking

[creativecreatorormaybenot]

Description

This expands on #2929: I think the collection reference was missing the == override and it also seems to make more sense to check if the pointers are equal rather than the paths, which are derived from the pointers. Essentially, this makes changing the Pointer behavior more easily.

Additionally, I added some tests that ensure that no regression occurs. The changelog entries that @IchordeDionysos added already reflect exactly what these changes do, so there is no need for another entry.

Related Issues

#2929

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• No, this is not a breaking change.

[creativecreatorormaybenot]

@Salakar Is the doc4 failing test caused by me? I do not exactly understand how it would cause that test to fail 😶

[Salakar]

@Salakar Is the doc4 failing test caused by me? I do not exactly understand how it would cause that test to fail 😶

Don't think so, we're aware of a potential race condition on the web tests that we're looking into.

[creativecreatorormaybenot]

@Ehesp done.

[Ehesp]

Looking good thanks, just a small change and should be good.