Create a simple QField plugin for switching baselayers

[spwoodcock]

Is your feature request related to a problem? Please describe.

• For Mapa das Periferias project.
• Users want a way to simply toggle the basemap to different layers, without going to the sidebar.
• We could add a button to the UI to swap between OSM and Satellite quickly.

Describe the solution you'd like

• Create a small global QField plugin that is installable.
• Instead of replicating functionality and having the ability to select any of the baselayers (this can be done with one extra click in the sidebar...).
• I think we should have two layers configured as connected to the plugin switcher.
• The user clicks the button and it simply toggles between the two layers, whether that is OSM & Satellite, OSM & OAM drone imagery, Satellite & offline drone imagery, etc.

[linear]

TECH-1115

[spwoodcock]

Related to #2940, as part of the project configuration, we want to enable switching to the bundled offline mbtiles layer. Probably default OSM + offline mbtiles toggle.

[spwoodcock]

Done in https://github.com/hotosm/qfield-next-theme-plugin