Support compression with external dictionary

Both Brotli and ZSTD support speeding up compression with a set of external dictionaries, trained from sample data.

It would be nice if our API could be made aware that (a) there is a dictionary involved in the compression and (b) how to upload/download that dictionary from/to client to/from server.

Review the repository's API definitions and current compression-related behavior before deciding how dictionary metadata and transfer should be represented. Done means the API specifies dictionary involvement and a complete client-to-server and server-to-client dictionary exchange for Brotli and ZSTD.

Written by the indexing model from the issue text.