Specifying implicit toolchain versions

[leostera]

Some toolchains do not have a clear version that are readable from configuration files. One example of this is protoc, that usually is expected to be installed globally via brew or apt.

To do this, we should add support in the Warpfile to define toolchain versions:

{
  "toolchains": {
    "protobuf": "3.21.12"
  }
}