Add `Keys` sync message #968

Job	Run time
Check code formatting	4s
Build (libsignal-service-actix, Rust stable)	1m 56s
Clippy (libsignal-service-actix)	1m 23s
Build (libsignal-service-actix, Rust beta)	1m 48s
Clippy (libsignal-service-hyper)	1m 11s
Build (libsignal-service-actix, Rust nightly)	4m 10s
Clippy (libsignal-service-hyper, unsend-futures)	1m 32s
Build (libsignal-service-hyper, Rust stable)	1m 44s
Clippy (libsignal-service)	1m 8s
Build (libsignal-service-hyper, Rust stable)	1m 27s
Clippy (libsignal-service, unsend-futures)	1m 14s
Build (libsignal-service-hyper, Rust beta)	1m 34s
Build (libsignal-service-hyper, Rust nightly)	3m 44s
Build (libsignal-service, Rust stable)	1m 5s
Build (libsignal-service, Rust stable)	1m 10s
Build (libsignal-service, Rust beta)	1m 13s
Build (libsignal-service, Rust nightly)	3m 20s
Build (libsignal-service-actix, Rust 1.75)	1m 51s
	31m 34s

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