[Feature] protocol: Add v3 multipart response parsing for proxy and body decompression

[Feature] protocol: Add v3 multipart response parsing for proxy and body decompression

Proxy forwarding now handles multipart/mixed responses from /checkv3:
parse result+body parts, decompress zstd, detect msgpack, and forward
rewritten body to milter. Self-scan v3 populates conn->results for
milter and Lua comparison scripts. rspamc client decompresses zstd
body parts returned by the server.