Moves most of https pacakge into /common/providers/https. (#915)

Moves most of https pacakge into /common/providers/https. The actual encoding/decoding/error logic etc. of callable functions will stay the same between v1 and v2, so they should be in a /common file. The rest of /v1 is about trigger options, cors, etc. onCall's CORS options are part of the v1 namespace because v2 adds a "cors" option to HttpsOptions. In the v2 API, we will default to `"cors": true`, but a customer could say `"cors": "my.firebase.app"` and we will respsect it. I wish there was a way to make this diff not so noisy. Most of this is cut & paste.

Author: inlined

spec/common/providers/https.spec.ts

@@ -287,7 +287,7 @@ describe('FunctionBuilder', () => {
  );
  });
 
- it('should not set a serviceAccountEmail if service account is set to `default`', () => {
+ it('should set a null serviceAccountEmail if service account is set to `default`', () => {
  const serviceAccount = 'default';
  const fn = functions
  .runWith({
@@ -296,7 +296,7 @@ describe('FunctionBuilder', () => {
  .auth.user()
  .onCreate((user) => user);
 
- expect(fn.__trigger.serviceAccountEmail).to.be.undefined;
+ expect(fn.__trigger.serviceAccountEmail).to.be.null;
  });
 
  it('should throw an error if serviceAccount is set to an invalid value', () => {