Show httpgd plot on attach

[renkun-ken]

What problem did you solve?

Close #832

This PR moves the separate httpgd request url into the attach request. If a httpgd device is open, then its url is also sent to session watcher on attach.

This PR also removes the code to send vsc.plot back to session watcher as it is already defined as r.session.viewers.viewColumn.plot in vscode settings.

(If you do not have screenshot) How can I check this pull request?

1. Open a workspace folder.
2. Create R terminal and session watcher attaches to the R session.
3. Run plot(rnorm(100)) and close the httpgd plot viewer.
4. Create another R terminal and session watcher attaches to the new R session.
5. Go to the first R terminal and run "Attach active terminal" or click the status bar item.
6. The first session should be attached and the httpgd plot viewer should also appear.

[ElianHugh]

Thanks for this - it works as expected on my laptop. I'd like to double check the interaction with Live Share, but won't have access until tomorrow

[renkun-ken]

In a live share session, it looks like the guest does not handle httpgd request in the first place. What's the expected behavior?

[ElianHugh]

Thank you for your patience - LGTM! Guests should have the host's httpgd session shared with them, but technically it is the host that is doing the attaching.