Adjust the ALT and SPEED of the vehicle during Auto mission flight

#### Issue details
Feature request:

It would be great if the Mission Planner would allow to adjust the speed and altitude of the Copter (or plane) in AUTO or GUIDED mode. For example when my QUAD is on the long mission, I am controlling it through the camera. The route is already defined in the FC. But what I need is to increase or decrease the ground speed (for example from 50% to 150% from the defined in WP_SPEED) and the altitude (-50m, -30m, -10m, +10,. +30m, +50m etc) so I can flexibly select the best speed and the altitude of my vehicle during the flight depends on the screen from the camera.

The speed and the altitude may be adjusted pretty smoothly so no spikes in the controlling values will be added.

Is this something possible?

#### Version
1.3.70

#### Platform
[ ] All
[ ] AntennaTracker
[X] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
QUAD

#### Hardware type
Holybro Kakute F7

#### Logs
_Please provide a link to any relevant logs that show the issue_

Start by reviewing Mission Planner's existing controls for Copter AUTO and GUIDED modes, since the issue names those modes but no files or tests. Define how users would adjust WP_SPEED-relative ground speed and altitude smoothly during flight, then verify the behavior on a Copter mission without control-value spikes.

Written by the indexing model from the issue text.