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/api/pod/util_test.go

@@ -841,10 +841,8 @@ func TestDropDynamicResourceAllocation(t *testing.T) {
 			},
 			ResourceClaims: []api.PodResourceClaim{
 				{
-					Name: "my-claim",
-					Source: api.ClaimSource{
-						ResourceClaimName: &resourceClaimName,
-					},
+					Name:              "my-claim",
+					ResourceClaimName: &resourceClaimName,
 				},
 			},
 		},