support custom exec command in server readiness probe

[bitnik]

This PR supports custom exec command in server readiness probe. If server.readinessProbe.execCommand is set, then it overwrites the default exec command in server readiness probe.

We need to be able to do this since we enforce mTLS by setting tls_require_and_verify_client_cert = "true" and so have to provide client certificates in readiness and liveness probes.

Note: I executed tests with docker run -it --rm -v "${PWD}:/test" hashicorpdev/vault-helm-test:latest bats /test/test/unit/server-statefulset.bats -f "server/standalone-StatefulSet: readiness".

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