update observability policy API to have minimum one target ref

[salonichf5]

Proposed changes

Write a clear and concise description that helps reviewers understand the purpose and impact of your changes. Use the following format:

Problem: User wants ObservabilityPolicy CRD should require a minimum of 1 TargetRef.

Solution: Add validation to have minimum one target ref for the ObservabilityPolicy CRDs.

Testing:

Tested manually using the example. Installed observabilityPolicy with on branch main, a target ref and ensuring everything works as expected.

Tested with specifying no target refs

kubectl apply -f - <<EOF
apiVersion: gateway.nginx.org/v1alpha1
kind: ObservabilityPolicy
metadata:
  name: coffee
spec:
  targetRefs: []
  tracing:
    strategy: ratio
    ratio: 75
    spanAttributes:
    - key: coffee-key
      value: coffee-value
EOF
The ObservabilityPolicy "coffee" is invalid:
* spec.targetRefs: Invalid value: 0: spec.targetRefs in body should have at least 1 items
* spec.targetRefs: Invalid value: "array": TargetRef Kind must be: HTTPRoute or GRPCRoute

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide specific feedback, add them here.

Closes #2524

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x] I have read the CONTRIBUTING doc
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked that all unit tests pass after adding my changes
• [x] I have updated necessary documentation
• [x] I have rebased my branch onto main
• [x] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes, please add a brief note that summarizes the change.

ObservabilityPolicy API requires minimum one target ref specified when creating a policy.