api/cluster: add join endpoint

api/cluster: add join endpoint

Add an endpoint to the API which allows to join an existing PVE
cluster by only using the API instead of CLI tools (pvecm).

Use a worker as this operation may need longer than 30 seconds.
With the worker we also get a task log entry/window for an UI for
free, allowing to give better feedback.

The join helper will be reused by the CLI handler in a later patch.
It is based on its behaviour, but swapped out the ssh parts with API
calls.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>