Add support for Proto 3 Optionals

[ummels]

Protobuf 3.12 has added support for optional fields, and with version 3.15 this is no longer an experimental feature.

Since the compiler plugin for rsocket-rpc-java depends on an older version of protoc, the plugin currently fails when confronted with a service definition that contains optional fields.

Please upgrade protoc and protobuf dependencies to a newer version so that the plugin can handle service definitions with optional fields. Since optional fields do not change the semantics of the code generated by this plugin, this should be enough in order to support optionals.

[iamceph]

This is a needed enhancement!