Keeper support for readiness endpoint (manifests)

[UnamedRus]

ClickHouse Keeper provides an HTTP interface to check if a replica is ready to receive traffic. It may be used in cloud environments, such as Kubernetes.

https://github.com/ClickHouse/ClickHouse/pull/55876

https://github.com/ClickHouse/clickhouse-docs/pull/1767/files#diff-a3037a0357709df221a285b2b5ca2df68b3f7d95e795f7409c594be469daf8a0R363

[Slach]

partially, solved in manual keeper deployment example https://github.com/Altinity/clickhouse-operator/pull/1501/files#diff-4004b397d3c55551afc9611863f1ee684f331b810518570329d0e3bdd79f88e4R303

@sunsingerus we actually need to discuss how exactly readiness probes shall works and how to implement scaleup / scaledown via reconfig

[sunsingerus]

readiness check introduced in 0.24.0 release check, please