feat: add support for detached cmds

[MichaelGoberling]

Ticket: https://jira.corp.adobe.com/browse/ACNA-4637

• Add support for running commands in the background, via detached: true option
• Add support for a new getCommand(execId) for "re-attaching" to a running background command
• Add support for cleanly closing the websocket connection on sandbox destroy, so there are no errors when users destroy sandboxes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[pru55e11]

same nit: if the server sends exec.exit before the exec.detached ack, the detached path mishandles it

(JS: wait() hangs; Python: raw TypeError on result["pid"])