Refactor /prepend and /append

[i25052i]

Describe the bug

• Currently the mechanism the /append waypoint uses to add a new Waypoint to the queue is unreliable. Change it to reconstruct the mission entirely like with a new mission.
  • See the PUSH section in client.py. Also, rename PUSH to something more sensible
• Currently the /prepend endpoint is very slow, because it 1. obtains the queue from MissionPlanner, 2. constructs the new queue, and 3. sends it back to MissionPlanner. If this logic was handled entirely within client.py, the endpoint, and the time it takes for the new queue to take effect, would be faster.
  • The mechanism for adding the prepended mission should be the same as above.

Reproduction steps

Call either endpoint

Expected behaviour

No response

Additional context

No response

[i25052i]

Also, update them to support waypoint commands like for POST /queue in #74