New service method: save_user_coords

[hxdimpf]

This is about adding a new service endpoint. The platforms (opencaching.xx) provide the capabilities to maintain a personal cache notes field for each cache. In addition with this there is also a capability to store user provided coordinates for instance the coordinates that are the result of solving a puzzle.

The OKAPI provides a service to retrieve the personal caches notes and also the user_coords. It also provides a service to update the personal cache notes but not the user coords.

I have implemented a new service to fill this gap. First I thought about extending the already existing service

:: services/caches/save_personal_notes

however, to make sure that this doesn't create any adverse side effects for existing client applications I decided to implement a new service:
:: services/caches/save_user_coords.

This issue will be referenced in an upcoming PR