GameServerSpec: Add PodTemplateSpec validation

[aLekSer]

Is your feature request related to a problem? Please describe.
There were 2 validation of PodTemplateSpec added recently - for Labels and Annotations. This approach could be generalised for all fields in a PodTemplateSpec.
This could run system into a failure state without informing user that he choose wrong pod template.
For instance fleet mentioned below with resources requests.memory > limits.memory is a mistake and no pod could be created from such template.
Fleet would start to fire up new GameServers infinitely as in aforementioned bugs with Labels.

Describe the solution you'd like
Use a standard Validation logic as could be found here for a ReplicaSet:
https://github.com/kubernetes/kubernetes/blob/52d7614a8ca5b8aebc45333b6dc8fbf86a5e7ddf/pkg/apis/apps/validation/validation.go#L674

Describe alternatives you've considered

Additional context
Fleet which does not signalise of a failure config:

apiVersion: "agones.dev/v1"
kind: Fleet
metadata:
  name: simple-udp-failure
spec:
  replicas: 2
  template:
    spec:
      ports:
      - name: default
        containerPort: 7654
      template:
        spec:
          containers:
          - name: simple-udp
            image: gcr.io/agones-images/udp-server:0.17
            resources:
              requests:
                memory: "66Mi"
                cpu: "20m"
              limits:
                memory: "64Mi"
                cpu: "20m"

[aLekSer]

Well, I have tried to reuse the version of ValidatePodTemplateSpec()
https://github.com/kubernetes/kubernetes/blob/52d7614a8ca5b8aebc45333b6dc8fbf86a5e7ddf/pkg/apis/apps/validation/validation.go#L674
But there are lots of issues with adding dependency into go modules: go get k8s.io/kubernetes/pkg/apis/core/validation@v1.13, go get don't work in this case.
There is a comment on how to reuse this code:
kubernetes/kubernetes#80316 (comment)

ValidatePodSpec contains lots of useful validations, and the idea of this ticket was to add once and support all these validations inline with kubernetes version updates.
https://github.com/kubernetes/kubernetes/blob/36981002246682ed7dc4de54ccc2a96c1a0cbbdb/pkg/apis/core/validation/validation.go#L3169

[aLekSer]

What could cause errors in creating Pods, as could be found in https://github.com/kubernetes/kubernetes/blob/master/pkg/apis/core/validation/validation.go#L3194 :

• nodeAffinity, podAffinity and podAntiAffinity
• dnsPolicy: "None" ("Required value: must provide dnsConfig when dnsPolicy is None")
• container fields like: lifecycle, startupProbe
  ...

[markmandel]

Worth exploring:
https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#rawextension

Another trick I saw (but now can't find again), that I thought was clever - run a dry run of a creation of a pod with the same details, and return any validation issues that it found.

What we might want to do though is keep a cache of of specs that have been marked valid, so we can skip hitting the API all at once for the dry run steps.

[markmandel]

Another trick @luna-duclos put me onto:

Using https://github.com/kubernetes-sigs/controller-tools/tree/master/cmd/controller-gen to generate the schema for the Pod:

Docs:
https://book-v1.book.kubebuilder.io/beyond_basics/generating_crd.html

Which can then be included into the schema for Game Server template. This actually sounds very promising!

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