Simplify ENUM names

[huan]

MESSAGE_TYPE_IMAGE -> IMAGE

See: https://cloud.google.com/apis/design/design_patterns#enum_default_value

[huan]

This can not be implemented due to the limitation of the gRPC itself does not permit.

See:

• protocolbuffers/protobuf#5425

.NET has a nice feature to rename all of them when running the generator (Protobuf enumerations), don't know if javascript has any similar tools.