[feature request] proto as toolchain plugin

At the moment we are able to define the proto version in moon via the `toolchains.yml`. The complete handling is implemented in moon itself. It would really great if proto can be a toolchain plugin because with this we would have some more possiblities 1) With an option we can define that all tools listed inside the `.prototools` shall be installed during the `moon sync workspace` stage Because at the moment only tools from a toolchain tier 3 will be installed automaticlly. 2) Or if the `moon workspace sync` can not be supported, due to toolchain detection we are able to create a common proto.yml inside the `.moon/tasks` folder which than can install the tools. What do you think about this idea? I can contribute this part.