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!)

[IvyZX]

Thanks for making this change! Can we have a test for this?

[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 (f2f76d9).
Report is 85 commits behind head on main.

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

Additional details and impacted files

@@          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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[IvyZX]

Thank you! Approved. You would need to pull and rebase for a trivial merge conflict.

[cool-RR]

@IvyZX Fixed.

[cool-RR]

@IvyZX Are we awaiting some automated process to get this merged?

[cool-RR]

@IvyZX Is this good for merging?