Summary:
master_fwd.h should only contain forwards, but it has master_replication.pb.h, which means
every file that includes master_fwd.h has to parse master_replication.pb.h too, which takes
significant time (according to ClangBuildAnalyzer).

This diff also changes the forward generator in gen_yrpc (out protobuf plugin) to forward declare
enums in `proto_foo.fwd.h`.

This diff also removes some unused proto includes that were generating compiler warnings.

**Upgrade / Downgrade Safety **
The proto changes are to unused includes.
Jira: DB-15911

Test Plan: existing tests

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D42766