feat: Add custom metadata support for IPC messages and RecordBatch

[rustyconover]

What's Changed

Decode and expose custom metadata from IPC message headers, propagating it through the reader to RecordBatch instances. This allows accessing per-batch metadata stored in Arrow IPC streams and files.

Extend RecordBatchWriter to support writing custom metadata to IPC messages, similar to PyArrow's write_batch(batch, custom_metadata=...).

Changes:

• Update Message.from() to accept optional metadata parameter
• Update Message.encode() to serialize custom metadata to FlatBuffers
• Add customMetadata parameter to RecordBatchWriter.write()
• Add mergeMetadata() helper that combines batch.metadata with the
  parameter (parameter takes precedence for duplicate keys)
• Add comprehensive integration tests for write/read round-trip

Usage:
writer.write(batch, new Map([['key', 'value']]));

[trxcllnt]

Looks good so far, just a few questions about the writer API.

[trxcllnt]

Suggested change

	return new Message(bodyLength, MetadataVersion.V5, MessageHeader.RecordBatch, header, metadata);
	return new Message(bodyLength, MetadataVersion.V5, MessageHeader.RecordBatch, header, header.metadata);

[trxcllnt]

Since the second metadata argument is only relevant if we're serializing a RecordBatch message, can we just use its metadata field instead?

Suggested change

	public static from(header: Schema | RecordBatch | DictionaryBatch, bodyLength = 0, metadata?: Map<string, string>) {
	public static from(header: Schema | RecordBatch | DictionaryBatch, bodyLength = 0) {

[trxcllnt]

If the Message reads the metadata from the RecordBatch, this can all be simplified.

[trxcllnt]

Then we can do this:

Suggested change

	const batch = createTestBatch();
	const metadata = new Map([['batch_id', '123'], ['source', 'test']]);
	const writer = new RecordBatchStreamWriter();
	writer.write(batch, metadata);
	const batch = createTestBatch();
	batch.metadata.set('batch_id', '123');
	batch.metadata.set('source', 'test');
	const writer = new RecordBatchStreamWriter();
	writer.write(batch);