Route actions prefix_rewrite and regex_rewrite should be a oneof

Title: Route actions prefix_rewrite and regex_rewrite should be a oneof

Description:

In the discussion for pull request #10050, it was suggested that the prefix_rewrite and regex_rewrite route actions be put under a oneof in the next api version, v4. However, it's too early to mark the proto files with that annotation now, so the code change for #10050 didn't add the annotation. This issue is to remember that this should be considered when the time is appropriate.