Add time estimate for RTL mission land

[KonradRudin]

Solved Problem

Currently, there is no time estimate available, when the RTL has selected the mission landing as destination.

Fixes #{Github issue ID}

Solution

• Add calculations for time estimate to the RTL mission landing, based on the time estimate in the RTL home landing.

Changelog Entry

For release notes:

Feature/Bugfix Add time estimate for RTL Mission landing

Test coverage

• Unit/integration test: ...
• Simulation/hardware testing logs: https://review.px4.io/