Publish protoc-gen-multi for combined plugins

[emcfarlane]

Combine multiple protoc plugin invocations into one with protoc-gen-multi. Publish a custom plugin which can invoke multiple protoc plugins with support for Generated SDKs. See README.md for details.

[mfridman]

I don't think we need extensive tests here, but if we wanted a bit of confidence we could pass a testdata code gen request and assert for the generated files:

.
└── connectrpc
    └── eliza
        └── v1
            ├── eliza.pb.go
            ├── eliza_grpc.pb.go
            └── eliza_vtproto.pb.go

[emcfarlane]

@mfridman added a test using testdata/request.json which is a pluginpb.CodeGeneratorRequest that will try invoke protoc-gen-go if available and check it gets some content.