feat(remote-config): custom signals support #796

Job	Run time
web-app-check	2m 55s
web-app-check	2m 55s
web (tests)	3m 24s
web (tests)	3m 27s
web-wasm (tests)	2m 54s
web-wasm (tests)	2m 54s
web (packages/cloud_firestore/cloud_firestore/example)	3m 25s
web (packages/cloud_firestore/cloud_firestore/example)	3m 25s
web-wasm (packages/cloud_firestore/cloud_firestore/example)	3m 24s
web-wasm (packages/cloud_firestore/cloud_firestore/example)	3m 24s
	32m 7s

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