add a protoc_plugin:generate CLI

[devoncarew]

This (WIP) PR adds a new CLI entrypoint to protoc_plugin to make it easier to generate the proto classes. It:

• discovers the protoc binary
• allows specifying the out directory
• allows specifying multiple proto search path dirs
• by default, automatically includes the google.protobuf protos (discovered from the protoc install path)
• is invoked via dart run protoc_plugin:generate

A wrapper around protoc to make it easier to generate Dart code from protobuf definitions.

usage: dart run protoc_plugin:generate [options] <proto files>
-h, --help                      Print this usage information.
    --version                   Display the protoc version in use.
    --proto-path=<directory>    Specify the directories in which to search for imported protos.
    --out=<directory>           The output directory for the generated code.
                                (defaults to "lib/src/gen")
    --grpc                      Generate gRPC service classes.

This needs more work, docs, testing, and discussion, so starting as a draft PR.