feat: add signet-host-rpc crate with RpcHostNotifier #468

Job	Run time
rust-library-base / Base Checks / Test Suite	7m 40s
rust-library-base / Feature Checks	4m 53s
rust-library-base / Base Checks / Docs	4m 15s
rust-library-base / Base Checks / doctest	6m 6s
rust-library-base / Base Checks / Rustfmt	43s
rust-library-base / Base Checks / Clippy	4m 9s
	27m 46s

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