AP_Mount: improve safety by disabling LRF at startup

[rmackay9]

During the review of #26516 we discussed concerns about user eye damage by having the laser range finder enabled at startup. While most gimbal manufacturers claim the long range laser included in the mounts are safe we think it would still be best to allow users to easily default the LRFs to off.

The linked PR adds support for users to control the on/off state with an auxiliary switch but it does not set the initial enabled/disabled state due to the initialisation order of RC channels (which are initialised first) and the Mount library (initialised later). The RC switch also only control the first mount meaning there would be no way to disable a 2nd mount's LRF.

A number of ideas were discussed:

1. Some gimbals include a setting to enable/disable the LRF at startup. The Siyi T30 however remembers its enabled/disabled state from the last time it was used (e.g. saved before shutdown)
2. We could use the RC_Channels::get_aux_cached() method to get around the RC and Mount init order issue (for the 1st gimbal)
3. We could add a MNTx_OPTION parameter bit to disable the LRF at startup
4. We could add a MNTx_OPTION to enable/disable the LRF with the vehicle although this would make verifying the LRF's working before takeoff