Sub: add an alt hold mode using the rangefinder

[jaylthegreat]

Feature request

Is your feature request related to a problem? Please describe.
Right now, there is just depth hold mode which usually just holds its depth from the surface; however, when a rangefinder is setup and healthy, it will try to maintain that rangefinder distance to the bottom rather than the depth.

Describe the solution you'd like
Maintaining distance from the bottom and maintaining distance from the bottom are totally distinct goals, and, as such, they should be in separate flight modes. I know that support for the rangefinder has been removed while it's being reworked, so this is a suggestion for once that is added back in.

Describe alternatives you've considered
The other solution might be to keep them both in depth hold mode, but then have boolean somewhere that tells the sub which distance to maintain.

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ X ] Submarine

Additional context
This page lays out that this intended for depth hold mode.

For context, I've been working on a project and have already implemented it into a separate mode on my fork of the stable branch (I called the mode terrain follow mode) . The more tedious deal (after reworking the rangefinder) is just integrating the new mode with QGC and MavProxy

[rmackay9]

As a reference, the way this is handled in copter is the range finder can be disabled using an auxiliary switch

[clydemcqueen]

An implementation: #23435

[clydemcqueen]

SURFTRAK is merged, so think we can close this bug. @Williangalvani

[Williangalvani]

Done in #23435

Thanks @clydemcqueen !