ExtraEnv

Extra environment the bridge hands to in-JVM generators, on top of what protoc sends.

protoc's plugin protocol has no room for this, so the bridge appends it as an unknown field on the serialized CodeGeneratorRequest and mirrors it in the child process environment. No Java or Scala class is generated for the message: the writer side must work without a protobuf-java of its own version on the classpath, so it serializes by hand (see ProtoUtils) and the reader side uses DynamicMessage.

Field number and variable name are deliberately kept byte-identical to protoc-bridge's (1020 is protobuf's globally reserved extension number for ScalaPB) so a third-party protoc plugin that already speaks this protocol keeps working.