g3proxy: add initial filter for SMTP extensions #1523

Job	Run time
Clippy	1m 16s
Build (stable)	2m 41s
Build vendored (vendored-openssl, g3proxy)	1s
Build (beta)	2m 7s
Build vendored (vendored-openssl, g3bench)	1s
Build (nightly)	2m 19s
Build vendored (vendored-openssl, g3tiles)	1s
Build vendored (vendored-openssl, g3fcgen)	1s
Build vendored (vendored-openssl, g3mkcert)	1s
Build vendored (vendored-openssl, g3keymess)	1s
Build vendored (vendored-tongsuo, g3proxy)	1s
Build vendored (vendored-tongsuo, g3bench)	1s
Build vendored (vendored-tongsuo, g3tiles)	1s
Build vendored (vendored-tongsuo, g3fcgen)	1s
Build vendored (vendored-tongsuo, g3mkcert)	1s
Build vendored (vendored-tongsuo, g3keymess)	1s
Build vendored (vendored-aws-lc, g3proxy)	2m 57s
Build vendored (vendored-aws-lc, g3bench)	2m 26s
Build vendored (vendored-aws-lc, g3tiles)	2m 32s
Build vendored (vendored-aws-lc, g3fcgen)	2m 8s
Build vendored (vendored-aws-lc, g3mkcert)	2m 11s
Build vendored (vendored-aws-lc, g3keymess)	2m 17s
Build vendored (vendored-boringssl, g3proxy)	1s
Build vendored (vendored-boringssl, g3bench)	1m 17s
Build vendored (vendored-boringssl, g3tiles)	1s
Build vendored (vendored-boringssl, g3fcgen)	1m 5s
Build vendored (vendored-boringssl, g3mkcert)	1s
Build vendored (vendored-boringssl, g3keymess)	1m 4s
Build vendored (vendored-c-ares, g3proxy)	1s
	26m 36s

Provide feedback