Add option to set custom proto path

[madsodgaard]

Allows the developer to supply a custom directory that will be used as the base of the protofiles.

{
    "invocations": [
        {
            "protoFiles": [
                "file1.proto",
                "file2.proto",
            ],
            "visibility": "public",
            "protoPath": "proto_repo_submodule/protos"
        }
    ]
}

This is useful if you are storing protofiles inside a Git repo and adding that as a submodule to other repos. Giving you a directory structure like:

MySwiftApp/
  - Sources
    - MyAppTarget
      - proto_repo_submodule
        - protos
          - file1.proto
          - file2.proto

and any proto files will be importing other proto files relative to that "protos" directory:

such as in file1.proto

import "file2.proto"