Add readiness endpoint for dedicated deployments #2401

Job	Run time
call_is_mergeable / Check if branch is mergeable	3s
build (3.8)	1m 31s
build (3.9)	1m 20s
build (3.10)	1m 32s
build (3.11)	1m 41s
build (3.12)	1m 38s
	7m 45s

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