Move istio/proxy protos into istio/api

Fixes istio/istio#35210.

This does two things:
* Move some protos we have in istio/istio that are manually generating
the go protos, using an outdated protobuf library and outdated proto.
* Move some protos we have in istio/proxy that are not generating any go
protos at all.

A next step will be to have istio/istio import these go libraries, and
istio/proxy to import these as proto_libraries.

buf.gen-golang.yaml

@@ -0,0 +1,8 @@
+# buf.gen.yaml sets up the generation configuration for all of our plugins.
+# Note: buf does not allow multi roots that are within each other; as a result, the common-protos folders are
+# symlinked into the top level directory.
+version: v1beta1
+plugins:
+- name: go
+  out: .
+  opt: plugins=grpc,paths=source_relative