Add `Keys` sync message #946

Job	Run time
Check code formatting	3s
Clippy (libsignal-service-actix)	1m 28s
Build (libsignal-service-actix, Rust stable)	1m 59s
Clippy (libsignal-service-hyper)	1m 24s
Build (libsignal-service-actix, Rust beta)	1m 46s
Clippy (libsignal-service-hyper, unsend-futures)	1m 10s
Build (libsignal-service-actix, Rust nightly)	4m 8s
Clippy (libsignal-service)	1m 7s
Build (libsignal-service-hyper, Rust stable)	1m 27s
Clippy (libsignal-service, unsend-futures)	1m 11s
Build (libsignal-service-hyper, Rust stable)	1m 43s
Build (libsignal-service-hyper, Rust beta)	1m 34s
Build (libsignal-service-hyper, Rust nightly)	3m 38s
Build (libsignal-service, Rust stable)	1m 8s
Build (libsignal-service, Rust stable)	1m 9s
Build (libsignal-service, Rust beta)	1m 19s
Build (libsignal-service, Rust nightly)	3m 15s
Build (libsignal-service-actix, Rust 1.75)	1m 56s
	31m 25s

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