Support socks5 proxy

### Is your feature request related to a problem? Please describe.
I want to use my tor client that expose a socks5 proxy, to connect to gRPC services exposed trough .onion endpoint

### Describe the solution you'd like

Add additional env var [here](https://github.com/grpc/grpc-node/blob/%40grpc/grpc-js%400.7.0/packages/grpc-js/src/http_proxy.ts#L45) something like `socks5_proxy`

### Describe alternatives you've considered
Using a client written in another language. ie. `grpc-go` has a custom net dialer you can pass down, so you can customize this.

### What I did

I tried doing this

```sh
export http_proxy="http://localhost:9150"
export grpc_proxy="http://localhost:9150"
export http_proxy="socks5://localhost:9150"
export http_proxy="socks5h://localhost:9150"
```

All do not work