FlightTask: Yawspeed slew rate for smooth motion

[MaEtUgR]

Describe problem solved by this pull request
This is a split out from the monster pr #12072. At the same time it provides a comparable solution to #13430.

Describe your solution

• I created a SlewRate class which implements a generic template slew rate with unit testing.
• I created a PositionLock class to handle reusable position and yaw lock logic for stick flying. It implements the yaw handling using the SlewRate which this pr is about and breaks out other already duplicate or well reusable related functions. This is part of the effort to make the whole flight task library more rely on reusable libraries than on overly complex inheriting.
• I adapted FlightTaskManualAltitude and FlightTaskManualPosition to call the reusable functions inside PositionLock.

Describe possible alternatives
A low pass filter like suggested in #13430.

Test data / coverage
SlewRate has unit testing, PositionLock I tested a lot in SITL and also multiple times on a real vehicle before it was 1:1 taken over from #12072. The calls based on current master are untested at the moment.

[bresch]

The lowpass method showed good results and there is no obvious reasons why we should try something different now. Closing for now and we can still re-open if we change our mind.