add websocket headers and proxy path support

[ktarplee]

This PR adds CLI flags for websocket headers and proxy path.

Some thoughts:

1. While the NATS API separates the proxy path from the server name, is that what we would want in the API? I think it would be cleaner to support a server URL and then extract the URL path from that and use it as the proxy path. -s wss://example.com/path/to/websocket as opposed to -s wss://example.com --proxy-path /path/to/websocket.
2. The NATS context config file should probably support setting the websocket headers and proxy path but that object seems to be defined in a the jsm.js project for some reason. It is not obvious to me why the config file for the NATS cli is in the jsm.go project.
3. I still need to fix the rtt command. It does some stuff differently that does not seem to work with websockets.

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[ripienaar]

We have other inflight PRs around this here nats-io/jsm.go#813 and here #1672 (comment) and nats-io/nats.go#2092

I have no idea what the correct approach is but seems there are some contradictory approaches here