This commit removes gogoproto.enumvalue_customname,
gogoproto.goproto_enum_prefix and gogoproto.enum_customname.
All of them make proto-generated Go code more idiomatic, but we already
don't use these enums in our external-surfacing types and they are anyway
not supported by Google's official toolchain (see #6564).
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
gogoproto.customtype is used to have go-digest.Digest instead of string.
While it is convinient, protoc-gen-go doesn't support the extension
and that blocks #6564.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
