proto: Reduce size of `Endpoint.handle` #5307

Job	Run time
test on freebsd	3m 55s
test on illumos	6m 33s
lint	1m 8s
msrv	1m 8s
audit	25s
test (ubuntu-latest, stable)	1m 33s
test-aws-lc-rs	1m 59s
test (ubuntu-latest, beta)	1m 38s
test on netbsd	4m 59s
test on solaris	6m 18s
test (macos-latest, stable)	1m 12s
test (windows-latest, stable)	2m 55s
test wasm32-unknown-unknown	54s
features (ubuntu-latest)	1h 22m 15s
features (macos-latest)	24m 7s
test-android (x86_64-linux-android, x86_64, 26)	3m 0s
features (windows-latest)	49m 35s
test-android (x86_64-linux-android, x86_64, 25)	2m 36s
test-android (i686-linux-android, x86, 26)	2m 58s
test-android (i686-linux-android, x86, 25)	2m 30s
	3h 21m 38s

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