[STRMCMP-944] Add proper validation for the sysctls field in securityContext

[mwylde]

A user created a flink config that had a map where an array was expected in the sysctls field of securityContext:

i.e., they had:

  securityContext:
    sysctls:
      net.ipv4.tcp_keepalive_time: "900"
      net.ipv4.tcp_retries2: "8"

when it should have been:

  securityContext:
    sysctls:
      - name: net.ipv4.tcp_keepalive_time
        value: "900"
      - name: net.ipv4.tcp_retries2
        value: "8"

This managed to take down the operator, which was spending all of its time logging this error:

E0507 00:24:06.711346       1 reflector.go:126] sigs.k8s.io/controller-runtime/pkg/cache/internal/informers_map.go:126: Failed to list *v1beta1.FlinkApplication: v1beta1.FlinkApplicationList.Items: []v1beta1.FlinkApplication: v1beta1.FlinkApplication.Spec: v1beta1.FlinkApplicationSpec.SecurityContext: v1.PodSecurityContext.Sysctls: []v1.Sysctl: decode slice: expect [ or n, but found {, error found in #10 byte of ...|sysctls":{"net.ipv4.|..., bigger context ...|,"savepointInfo":{},"securityContext":{"sysctls":{"net.ipv4.tcp_keepalive_time":"900","net.ipv4.tcp_|...

This PR adds validation of the sysctls field to the CRD to prevent such configs from being accepted by the API.

With this change, users will get the following error when they try to create the bad config above:

The FlinkApplication "operator-test-app" is invalid: spec.securityContext.sysctls: Invalid value: "object": spec.securityContext.sysctls in body must be of type array: "object"

The deeper issue here is that in the source code we re-use the existing Kubernetes API definitions for objects like securityContext. But we're prevented by kubernetes/kubernetes#62872 from also using the existing schemas, so we are forced to duplicate them. When we miss fields like we did here, it creates the potential for users to submit invalid configuration that can take down the operator.