Add current_dir option to prost_build::Config

[kriswuollett]

As seen previously in #222, proto path options may be specified from a different relative location, e.g., workspace root, while the build process is done from the working directory of the Rust package.

Adding a current_dir option to prost_build::Config will make it easier to easier to compile Rust protos where the proto files are in a source tree separate from the rust code without needing to specify a relative path from the rust package dir to the protos dir, for example in a repo like:

.
├── Cargo.lock
├── Cargo.toml
├── README.md
├── crates
│   ├── my-package
│       ├── Cargo.toml
│       ├── build.rs
│       └── src
│           └── lib.rs
├── protos
│   └── my_package
│       └── example.proto
└── protos.json

where the build.rs, and its dependencies, gets protobuf information from protos.json that may look like:

{
  "packages": [
    {
      "name": "my-package",
      "protobuf": {
        "sources": [
          {
            "include": "./protos",
            "protos": [
              "my_package/example.proto"
            ]
          }
        ]
      }
    }
  ]
}

It is awkward to specify that the include dir in the example would need to be ../../protos just for Rust.