Add host checking, restrict URLs

[samtstern]

Description

Fixes #1722
Fixes #1773

Scenarios Tested

functions

exports.helloWorld = functions.https.onRequest(async (req, res) => {
    res.json({
        hello: 'world',
        date: new Date()
    });
});

exports.firestoreWriter = functions.https.onRequest(async (req, res) => {
    await admin.firestore().doc('foo/bar').set({
        date: new Date()
    });

    res.json({ ok: 'ok' })
});

exports.firestoreReader = functions.firestore.document('foo/bar').onWrite(async (change, ctx) => {
    console.log("Firestore Write:", JSON.stringify(change.after.data()));
});

firebase.json

{
  "emulators": {
    "functions": {
      "host": "0.0.0.0",
      "port": 5001
    },
    "database": {
      "port": 9000
    }
  }
}

Start the emulators:

$ npx firebase emulators:start --only functions,firestore
i  Starting emulators: ["functions","firestore"]
✔  functions: Using node@10 from host.
✔  functions: Emulator started at http://0.0.0.0:5001
i  firestore: Serving ALL traffic (including WebChannel) on http://localhost:8080
⚠  firestore: Support for WebChannel on a separate port (8081) is DEPRECATED and will go away soon. Please use port above instead.
i  firestore: Emulator logging to firestore-debug.log
✔  firestore: Emulator started at http://localhost:8080
i  firestore: For testing set FIRESTORE_EMULATOR_HOST=localhost:8080
i  functions: Watching "/tmp/tmp.ITmKYw5141/functions" for Cloud Functions...
✔  functions[helloWorld]: http function initialized (http://0.0.0.0:5001/fir-dumpster/us-central1/helloWorld).
✔  functions[firestoreWriter]: http function initialized (http://0.0.0.0:5001/fir-dumpster/us-central1/firestoreWriter).
✔  functions[firestoreReader]: firestore function initialized.
✔  All emulators started, it is now safe to connect.

Run some functions:

$ http http://0.0.0.0:5001/fir-dumpster/us-central1/firestoreWriter
HTTP/1.1 200 OK
connection: keep-alive
content-length: 11
content-type: application/json; charset=utf-8
date: Sat, 09 Nov 2019 00:19:41 GMT
etag: W/"b-2F/2BWc0KYbtLqL5U2Kv5B6uQUQ"
x-powered-by: Express

{
    "ok": "ok"
}

Logs look good:

i  functions: Beginning execution of "firestoreWriter"
i  functions: Finished "firestoreWriter" in ~1s
i  functions: Beginning execution of "firestoreReader"
>  Firestore Write: {"date":{"_seconds":1573258780,"_nanoseconds":854000000}}
i  functions: Finished "firestoreReader" in ~1s

Sample Commands

[coveralls]

Coverage remained the same at 66.248% when pulling 72b6ec2 on ss-functions-host into 5f77df9 on master.

[bkendall]

One test nit, otherwise looks fine.