Expose User.changes

[nirinchev]

Adds User.changes which allows developers to subscribe for notifications whenever the user changes.

Depends on realm/realm-core#7302.

[coveralls-official]

Pull Request Test Coverage Report for Build 7756551836

Warning: This coverage report may be inaccurate.

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A quick fix for this PR: rebase it; your next report should be accurate.

• -3 of 32 (90.63%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 87.41%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/src/user.dart	18	21	85.71%

Files with Coverage Reduction	New Missed Lines	%
lib/src/realm_object.dart	1	86.24%

Totals
Change from base Build 7741947640:	0.05%
Covered Lines:	3770
Relevant Lines:	4313

---

💛 - Coveralls

[nielsenko]

Maybe use: git describe --abbrev=5 to name the core version, ie.

❯ git describe --abbrev=5
v13.26.0-13-gd12c3 

so we/they know it is not a clean core-release we are using.