feat: add custom_metadata support to RecordBatch with IPC read/write

[rustyconover]

Which issue does this PR close?

• Closes Support per-batch custom_metadata on RecordBatch (IPC Message field) #9444.

What changes are included in this PR?

Add per-batch custom_metadata to RecordBatch, matching the custom_metadata field on the IPC Message flatbuffer envelope. This allows attaching per-batch metadata separate from schema-level metadata, bringing parity with PyArrow's write_batch(custom_metadata=...) API (available since PyArrow v11.0.0).

Changes:

• Add custom_metadata: HashMap<String, String> field to RecordBatch with custom_metadata(), custom_metadata_mut(), with_custom_metadata(), and into_parts_with_custom_metadata() accessors
• IPC writer: serialize custom_metadata to Message flatbuffer
• IPC reader: extract custom_metadata from Message at FileDecoder, StreamReader, and StreamDecoder call sites
• arrow-flight: extract and propagate custom_metadata in flight_data_to_arrow_batch
• arrow-select: preserve custom_metadata through filter_record_batch and take_record_batch
• Metadata preserved through slice(), project(), normalize(), with_schema(), and remove_column()
• PyArrow-generated test data for cross-language interop validation

Are these changes tested?

Yes there are tests in the PR.

Are there any user-facing changes?

There are no breaking changes.

Written with AI assistance; all changes reviewed by the author.

[rustyconover]

This actually doesn't have anything to do with arrow-flight