proto: Make Connection externally use SideArgs #5220

Job	Run time
test-aws-lc-rs	1m 40s
test on netbsd	5m 1s
test on freebsd	3m 54s
test on solaris	5m 19s
audit	25s
features (ubuntu-latest)	1h 24m 27s
features (macos-latest)	29m 29s
lint	1m 4s
features (windows-latest)	49m 36s
msrv	1m 3s
test (ubuntu-latest, stable)	1m 32s
test-android (x86_64-linux-android, x86_64, 26)	2m 53s
test (ubuntu-latest, beta)	1m 19s
test-android (i686-linux-android, x86, 26)	2m 58s
test (macos-latest, stable)	1m 48s
test (windows-latest, stable)	3m 59s
test wasm32-unknown-unknown	48s
test on illumos	6m 30s
	3h 23m 45s

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