Corridor mapping - linear structures mapping

#### Issue details
The proposal is to carry out mapping along linear structures: highways, ducts, railways, etc.

The idea is to (1) indicate an alignment to be mapped (the axis of the linear structure); (2) indicate the mapping width along this axis; (3) collect the photos with percentage coverage values (laeral and sequential).

This question is open on two other related topics:

1. https://discuss.ardupilot.org/t/corridor-mapping/21398
2. https://github.com/mavlink/qgroundcontrol/issues/5695

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

#### Airframe type
Flying wing / hexacopter

#### Hardware type
Pixhawk (on my case)

Start by reviewing the issue and the linked Corridor Mapping and QGroundControl discussions to clarify the required workflow and coverage definitions. Then locate Mission Planner's mission-planning entry points for alignment, corridor width, and photo coverage. Done should include an agreed implementation scope for highways, ducts, or railways and validation of lateral and sequential coverage values.

Written by the indexing model from the issue text.