Support of required fields

[junm-cloudnatix]

protoc-gen-grpc-gateway-ts always put ? suffix for the fields, as fields in proto3 are basically optional. But I want the generator to support omitting the ? suffix when it's marked as non-optional in a way.

An idea is to consider a field is a required and If the field is specified as google.api.field_behavior = REQUIRED annotation (cf. https://github.com/googleapis/googleapis/blob/master/google/api/field_behavior.proto#L61 and an example https://github.com/googleapis/googleapis/blob/master/google/logging/v2/logging.proto#L154).