DRA: remove "source" indirection from v1 Pod API

This makes the API nicer:

    resourceClaims:
    - name: with-template
      resourceClaimTemplateName: test-inline-claim-template
    - name: with-claim
      resourceClaimName: test-shared-claim

Previously, this was:

    resourceClaims:
    - name: with-template
      source:
        resourceClaimTemplateName: test-inline-claim-template
    - name: with-claim
      source:
        resourceClaimName: test-shared-claim

A more long-term benefit is that other, future alternatives
might not make sense under the "source" umbrella.

This is a breaking change. It's justified because DRA is still
alpha and will have several other API breaks in 1.31.

pkg/controller/resourceclaim/controller_test.go

@@ -647,10 +647,8 @@ func reserveClaim(claim *resourcev1alpha2.ResourceClaim, pod *v1.Pod) *resourcev
 
 func makePodResourceClaim(name, templateName string) *v1.PodResourceClaim {
 	return &v1.PodResourceClaim{
-		Name: name,
-		Source: v1.ClaimSource{
-			ResourceClaimTemplateName: &templateName,
-		},
+		Name:                      name,
+		ResourceClaimTemplateName: &templateName,
 	}
 }