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.