[release-1.8] update CUSTOM action in AuthorizationPolicy

[istio-testing]

Manual cherrypick required.

#1698 failed to apply on top of branch "release-1.8":

Applying: update external action API
Using index info to reconstruct a base tree...
M	kubernetes/customresourcedefinitions.gen.yaml
M	mesh/v1alpha1/config.pb.go
M	mesh/v1alpha1/config.proto
M	mesh/v1alpha1/istio.mesh.v1alpha1.gen.json
M	proto.lock
M	python/istio_api/mesh/v1alpha1/config_pb2.py
M	security/v1beta1/authorization_policy.pb.go
M	security/v1beta1/authorization_policy.proto
Falling back to patching base and 3-way merge...
Auto-merging security/v1beta1/authorization_policy.proto
Auto-merging security/v1beta1/authorization_policy.pb.go
Auto-merging python/istio_api/mesh/v1alpha1/config_pb2.py
CONFLICT (content): Merge conflict in python/istio_api/mesh/v1alpha1/config_pb2.py
Auto-merging proto.lock
Auto-merging mesh/v1alpha1/istio.mesh.v1alpha1.gen.json
Auto-merging mesh/v1alpha1/config.proto
CONFLICT (content): Merge conflict in mesh/v1alpha1/config.proto
Auto-merging mesh/v1alpha1/config.pb.go
CONFLICT (content): Merge conflict in mesh/v1alpha1/config.pb.go
Auto-merging kubernetes/customresourcedefinitions.gen.yaml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 update external action API
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".