rpc v2: chainhead support multiple finalized block hashes in `FollowEvent::Initialized` #4289

Job	Run time
Check unused dependencies	21s
Cargo fmt	15s
Cargo clippy (WASM)	1m 0s
Cargo clippy	4m 44s
Cargo check	12m 3s
Check documentation and run doc tests	6m 12s
Cargo check (WASM)	1m 43s
Test (no_std)	1m 2s
Test (Unstable Backend)	9m 6s
Test (Light Client)	5m 53s
Test (WASM)	6m 23s
Test (Native)	9m 20s
	58m 2s

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