Add REFLECT padding to convolution layer

[sarlinpe]

What does this PR do?

Add a REFLECT padding mode to the linen covolution layer.

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!)

[sarlinpe]

@cgarciae It would be great to receive your review, thank you!

[cgarciae]

Thanks @sarlinpe for doing this! Looks good.
Can you make the same update for nnx.Conv? We try to keep them in sync.

[sarlinpe]

Done, PTAL.

[sarlinpe]

@cgarciae Friendly ping, thank you!