GH-33390: [R] Field-level metadata

[max-romagnoli]

Rationale for this change

• field() in R unlike Python does not support field-level metadata.

What changes are included in this PR?

• New active bindings on Field: $HasMetadata, $metadata
• New methods on Field: $WithMetadata(), $RemoveMetadata()
• New check_metadata parameter in Field$Equals(), defaulting to FALSE
• Tests for the above

Are these changes tested?

• Yes, unit tests have been added, the functionality was also tested locally in R

Are there any user-facing changes?

• Yes, metadata= now works without throwing an error.
• No breaking changes, since this was already included but errored before and parameter order was not changed in the implementation.

• GitHub Issue: [R] Allow setting field metadata #33390

[github-actions]

⚠️ GitHub issue #33390 has been automatically assigned in GitHub to PR creator.

[thisisnic]

Thanks for making this PR! I think it just needs a few more tests:

• WithMetadata(NULL) - sets empty metadata
• a roundtrip test to check we can save and load it and it matches
• (what happens if there are duplicate keys?)
• nested type, e.g. a field with struct type containing their own metadata