Automatically refresh the CA and certificate/credentials without shutting down the server

### Is your feature request related to a problem? Please describe.

GRPC-js is using http2 to make the underlying connection. There's a function called `setupServer` in `src/server.ts` that does this.

I want to be able to renew the certificates used by GRPC without interrupting existing connections. There is a function in node for this: https://stackoverflow.com/a/57197663/582917

Basically you use it like:

```js
server.setSecureContext({
ca: fs.readFileSync('chain.pem'),
cert: fs.readFileSync('cert.pem', 'utf8'),//fullchain
key: fs.readFileSync('privkey.pem', 'utf8')
})
```

### Describe the solution you'd like

Ideally this function would be exposed by the grpc server object as well, so we can update the certificates with zero downtime.

I'm not sure if this would work if the server was created with non-secure credentials, because that uses `http.createServer` and that may not have that function.

The `setSecureContext` is part of the `tls.Server` class in Nodejs.

But if that's the case, we can just throw an exception in that case.

Or alternatively a way to access the underlying server object to do this.