Update gRPC library in dgraph-js to use gRPC-js

[all-seeing-code]

1. Extend support to connect to dgraph client via Slash endpoints and apiKey

const clientStub = new dgraph.clientStubFromSlashGraphQLEndpoint(
  graphQlEndpoint,
  apiKey
);
const dgraphClient = new dgraph.DgraphClient(clientStub);

2. Migrate from grpc npm module to grpc-js in lieu of the deprecation of the former.

This change is 

[vardhanapoorv]

Minor comments. Add an example for this in the examples folder and also add a section in README regarding the usage. Later let's see if we can add a test for this.

Reviewed 2 of 17 files at r1, 2 of 3 files at r2, 4 of 10 files at r3, 8 of 12 files at r4, 2 of 2 files at r5, 2 of 2 files at r6, 8 of 8 files at r7.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @anurags92 and @gja)

---

package.json, line 53 at r7 (raw file):

  "dependencies": {
    "@grpc/grpc-js": "^1.1.7",
    "@types/url-parse": "^1.4.3",

"@types/url-parse" can go in devDependencies

---

examples/tls/index.js, line 12 at r7 (raw file):

    //     $ dgraph cert -n localhost
    //     $ dgraph cert -c user
    console.log(path.join(__dirname, "tls", "ca.crt"));

log statement can be removed

---

src/clientStubFromSlash.ts, line 5 at r7 (raw file):

import { DgraphClientStub } from "./clientStub";

const PORT = "443";

can be lowercase