proto: Connection side enum #5181

Job	Run time
test on netbsd	5m 2s
test on freebsd	3m 58s
test on solaris	3m 10s
test on illumos	7m 30s
lint	34s
msrv	58s
test (ubuntu-latest, stable)	1m 27s
test (ubuntu-latest, beta)	1m 12s
test-aws-lc-rs	1m 45s
test (macos-latest, stable)	1m 9s
test wasm32-unknown-unknown	50s
test (windows-latest, stable)	4m 29s
audit	25s
features (ubuntu-latest)	1h 20m 40s
test-android (x86_64-linux-android, x86_64, 26)	2m 57s
features (macos-latest)	23m 36s
test-android (i686-linux-android, x86, 26)	2m 48s
features (windows-latest)	49m 37s
	3h 12m 7s

Provide feedback