Support message level custom_metadata on DictionaryBatch (IPC Message field)

[ennuite]

Describe the enhancement requested

Analogous to #1035 but for DictionaryBatch.

The Arrow IPC format supports Message level custom_metadata, see https://arrow.apache.org/docs/format/Columnar.html#custom-application-metadata and https://github.com/apache/arrow/blob/main/format/Message.fbs#L154

The DictionaryBatch in this implementation currently has no support for this field.

[ennuite]

As of right now I don't see much interest from the community in having this so I won't be working on it. If anyone is interested please let me know and I'll try to provide an implementation.