Allow metadata pass-through in flax.struct.field

[cool-RR]

What does this PR do?

Allows passing through metadata when creating a flax.struct.field. This is useful when you want to add some metadata to your field to be read later.

Checklist

• [ ] This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• [ ] This change is discussed in a Github issue/ discussion (please add a link).
• [ ] The documentation and docstrings adhere to the documentation guidelines.
• [ ] This change includes necessary high-coverage tests. (No quality testing = no merge!)

[cool-RR]

@IvyZX Pushed a test, let me know if anything else is needed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (31adb00) to head (62b15bf). Report is 78 commits behind head on main.

Files	Patch %	Lines
flax/struct.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4056 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 106 107 +1 Lines 13582 13824 +242 ====================================== - Misses 13582 13824 +242 ```

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