feat: Support listDocuments()/listCollections() via the Firestore Emulator

[ryanpbrewster]

These custom headers can be passed in via an customHeaders field in the constructor, and are plumbed all the way through to the gRPC metadata in CallOptions.

With this change it is much simpler to connect to the Firestore emulator:

const grpc = require("@grpc/grpc-js");
const Firestore = require("@google-cloud/firestore");
const firestore = new Firestore({
  projectId: 'firestore-emulator-sample',
  servicePath: 'localhost',
  port: 8080,
  sslCreds: grpc.credentials.createInsecure(),
  customHeaders: {
    "Authorization": "Bearer owner"
  }
});

I thought this was the right level of abstraction: not too general (only constant headers provided at instantiation time) but not too specific (allows arbitrary headers, rather than only the Authorization header, or some special purpose emulator field).

[schmidt-sebastian]

Nice! LGTM.