Support gRPC-over-HTTPS using HTTPS proxy.

PR #1243 introduced proxy support allowing for gRPC-over-HTTP, i.e. an unencrypted proxy. While gRPC is encrypted by means of HTTP/2, and remains so when tunneled, the initial connection to the proxy is not. See https://github.com/grpc/grpc-node/blob/%40grpc/grpc-js%400.7.0/packages/grpc-js/src/http_proxy.ts#L133-L145

The initial HTTP `CONNECT` call contains two pieces of potentially private/secret data:
- The intended host to proxy
- The username/password when using an authenticated proxy

For these reasons it may be desirable to encrypt the `CONNECT` traffic via HTTPS.

Additionally, the current implementation ignores the protocol defined in the proxy configuration (only `PROXY_INFO.address` is used). When `https` is defined, this client will _still attempt_ to connect over `http`. If the proxy is expecting to negotiate TLS on the defined port (`URL.host`), it will error unexpectedly.

### Solution
Detect the presence of HTTP or HTTPS via `URL.protocol` and make the `CONNECT` request accordingly:
```ts
// pseudo-code
import * as http from 'http';
import * as https from 'https';

interface ProxyInfo {
address?: string;
creds?: string;
protocol?: string;
}

const schemeLib = PROXY_INFO.protocol === 'http' ? http : https; // secure by default
const request = schemeLib.request(options);
request.once('connect', (res, socket, head) => {/*...*/});
```

### Alternative
Be explicit about the lack of HTTPS support by both documenting, and detecting the usage of HTTPS and subsequently throwing an informative error.