Add `Keys` sync message #949

Job	Run time
Check code formatting	2s
Build (libsignal-service-actix, Rust stable)	1m 55s
Clippy (libsignal-service-actix)	1m 31s
Build (libsignal-service-actix, Rust beta)	1m 47s
Clippy (libsignal-service-hyper)	1m 15s
Build (libsignal-service-actix, Rust nightly)	4m 18s
Clippy (libsignal-service-hyper, unsend-futures)	1m 12s
Build (libsignal-service-hyper, Rust stable)	1m 31s
Clippy (libsignal-service)	1m 7s
Build (libsignal-service-hyper, Rust stable)	1m 29s
Clippy (libsignal-service, unsend-futures)	1m 19s
Build (libsignal-service-hyper, Rust beta)	1m 28s
Build (libsignal-service-hyper, Rust nightly)	3m 28s
Build (libsignal-service, Rust stable)	1m 15s
Build (libsignal-service, Rust stable)	1m 14s
Build (libsignal-service, Rust beta)	1m 16s
Build (libsignal-service, Rust nightly)	3m 9s
Build (libsignal-service-actix, Rust 1.75)	1m 51s
	31m 7s

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