feat: add signet-host-rpc crate with RpcHostNotifier #571

Job	Run time
rust-library-base / Feature Checks	5m 28s
rust-library-base / Base Checks / Clippy	1m 52s
rust-library-base / Base Checks / doctest	3m 38s
rust-library-base / Base Checks / Rustfmt	44s
rust-library-base / Base Checks / Docs	5m 2s
rust-library-base / Base Checks / Test Suite	5m 29s
	22m 13s

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