GH-559: [Java] Add FixedSizeBinary support to ComplexCopier

[Maria-Berta]

Rationale for this change

ComplexCopier did not support copying FixedSizeBinary columns nested in List, Map, Struct, or top-level contexts, throwing UnsupportedOperationException.

What changes are included in this PR?

Adds FIXEDSIZEBINARY cases to:

• getListWriterForReader
• getStructWriterForReader
• getMapWriterForReader
• the main copy() switch

Are these changes tested?

Yes:

• testCopyListOfFixedSizeBinary — copying a List
• testCopyStructOfFixedSizeBinary — copying a Struct field of type FixedSizeBinary

Full TestComplexCopier suite (22 tests) passes with no regressions.

Note: I attempted to add equivalent coverage for Map values of type FixedSizeBinary, but ran into a pre-existing limitation unrelated to this fix — MapWriter/ListWriter's no-arg fixedSizeBinary() delegates to NullableStructWriter.fixedSizeBinary(String), which only looks up an existing child writer and never creates one. The byteWidth-aware overload that does create the vector isn't reachable through the public MapWriter/ListWriter interface. This appears to be a gap in the writer codegen itself rather than something ComplexCopier can work around, so I've left it untested here — happy to open a follow-up issue if that's useful, or take a stab at it if maintainers think it's in scope for this PR.

Closes #559

[Maria-Berta]

Hi @jbonofre @laurentgo @lidavidm @wgtmac — checking in on this PR.
I don't have permission to add labels myself as an external
contributor. Could someone add bug-fix (or whichever fits best)
when you have a chance so the "Ensure PR is labeled" check can pass?
Happy to make any other changes needed too. Thanks!