[Core Feature] Customize PodSpec for Pod tasks

[katrogan]

Motivation: Why do you think this is important?
It should be possible to customize a pod spec for Pod Tasks further than just defining the pod spec. For example, users should be able to specify labels and annotations.

Goal: What should the final outcome look like, ideally?
Pod tasks expose an interface for modifying most components of a PodSpec

Describe alternatives you've considered

• Using the custom field for task plugins, however this is currently reserved for the marshalled pod spec
• Adding to the task config however this isn't well-suited for structured data (e.g. labels and annotations)

[Optional] Propose: Link/Inline OR Additional context
Add an alternative target: google.protobuf.Struct pod_spec = X; which represents a marshalled Pod [see below for definition] which will be used alongside the existing task definition to run the Pod flyte task.

[EngHabu]

I think the additional target should be something like:

message K8sPod {
   K8sObjectMetadata metadata = 1;
   google.protobuf.Struct pod_spec = 2;
}

message K8sObjectMetadata {
  map<string, string> labels = 1;
  map<string, string> annotations = 2;
}

// and in taskTemplate:

oneof target {        
  Container container = 6;    
  K8sPod k8s_pod = 7;
}

So we can control how much of K8s Pods do we want to expose to users to set?