[ext_proc] cleanups in ext_proc.cc

*Title*: cleanups in ext_proc.cc, pr 1

*Description*:
Let's do some cleanups in ext_proc.cc, there are quite a bit unnecessary C&P and helper datastructures that could be cleaned up.
If config comes from XDS, an good assumption is that filter instance can just point to the filter config without life-cycle concern, but I saw in action.cc is using a temp protobuf instance to instantiate the filter.

the protobuf is now pinned to FilterConfig so that we can reference to the proto directly instead making per-request copies.