[Consensus Observer] Make subscription creation asynchronous. #65530

Job	Run time
file_change_determinator	11s
rust-doc-tests	5m 5s
rust-consensus-only-unit-test	0s
rust-unit-tests	0s
rust-consensus-only-smoke-test	0s
rust-cargo-deny	1m 48s
general-lints	3m 26s
rust-smoke-tests	5m 55s
rust-lints	3m 13s
rust-cryptohasher-domain-separation-check	0s
rust-targeted-unit-tests	5m 55s
rust-check-merge-base	5m 29s
rust-build-cached-packages	2m 3s
	33m 5s

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