[Feature] rspamc: Add --msgpack flag for v3 protocol

[Feature] rspamc: Add --msgpack flag for v3 protocol

Add --msgpack option to rspamc that sends metadata as msgpack instead
of JSON and requests msgpack responses when using --protocol-v3.

The client serializes metadata via UCL_EMIT_MSGPACK, sets the metadata
part Content-Type to application/msgpack, and sends Accept:
application/msgpack so the server returns results in msgpack format.

Add functional tests for rspamc v3 with zstd compression, httpcrypt
encryption, msgpack metadata, and encrypted+msgpack combinations.