WP mission waypoint enforce altitude option

[breadoven]

Provides an option to force mission waypoint altitude to be achieved before moving on to the next waypoint. If the set altitude hasn't been achieved when the craft arrives at the waypoint it will hold and adjust the altitude as required with a target margin of 100cm. Planes use a spiral loiter whilst changing altitude. Option is set for the whole mission and applicable to basic and timed hold waypoints.

Tested on fixed wing in flight. Multirotor still needs flight testing.

Closes #6336.

[breadoven]

Tested on a multicopter and works fine. However, it needs #7645 to prevent toilet bowling like instability at the hold position during height adjustment. Not sure if this was an issue before for timed hold with nav_mc_wp_slowdown set OFF, looking at the code I'd have thought it was @DzikuVx ?

[breadoven]

Any objections to merging this @DzikuVx @stronnag ?

[stronnag]

+1 Nice (and belated) feature. My only nit-pick is that the user visible setting might be more intuitively named nav_wp_enforce_altitude or nav_wp_force_altitude rather than nav_wp_capture_altitude. But that's trivia.

[breadoven]

Yes I wasn't entirely sure about capture. Happy to change it to "enforce".