Add support for GRASS addons

[AlisterH]

Feature description

Mailing list and forum traffic shows that people often want to use GRASS addons in QGIS, including some asking how they could implement them in a plugin.

I propose what is needed, in rough order of priority:

1. Add support for installing and uninstalling addons (I will make a pull request for this in a moment)
2. Make the GRASS processing provider search a directory in the QGIS profile folder for algorithm definitions (in addition to the folder where definition files shipped with QGIS are kept, which is usually read-only).
3. Perhaps provide an easy way for a QGIS plugin to check whether a GRASS extension is installed (and then install it) when the plugin is enabled. Note that it might not be wise for a plugin to uninstall a GRASS extension, in case the user (or another plugin) needs it.
4. Figure out how to manage the risk of competing versions of an algorithm definition overwriting each other (essentially the same issue as for processing models - see my question at https://www.mail-archive.com/qgis-developer@lists.osgeo.org/msg55031.html)
5. Perhaps provide a way for people to share algorithm definitions.

Maybe the best way to deal with 3 and 5 is for people to create simple plugins simply to share algorithms, and other plugins define these plugins as a dependency.

Additional context

No response