[Ruby] Add support for custom metadata in Message

[kou]

Describe the enhancement requested

Message can attach custom metadata:

arrow/format/Message.fbs

Lines 152 to 157 in 61ef672

	table Message {
	version: org.apache.arrow.flatbuf.MetadataVersion;
	header: MessageHeader;
	bodyLength: long;
	custom_metadata: [ KeyValue ];
	}

Component(s)

Ruby

[kou]

Issue resolved by pull request 49621
#49621