[Feat] g.extension: enable installation of (python) addons locally on Windows

[ecodiv]

Is your feature request related to a problem? Please describe.
The g.extension module does offer the option to provide an alternative ULR to an addon repository. However, this does not work for GRASS GIS on Windows. This makes sharing quick updates (e.g., bug fixes) or addons outside the official repository with Window users difficult.

Describe the solution you'd like
Make it possible to install addons locally, or make it possible to point to another server than the default http://wingrass.fsv.cvut.cz/. I.a.w., make the URL parameter also work for Window users.

I tested this by replacing in the g.extension code the http://wingrass.fsv.cvut.cz/ with a URL to a site where I shared some addons, and using a path to a local folder with (Python) addons properly packaged in a zip file. In both cases, I could install the addon from that URL or folder without problem.

Describe alternatives you've considered

• One option is if the user sets the url parameter, the base_url variable (line 1601) will be constructed using that URL instead of the default one. That probably needs an extra note in the help file about the requirements of how the addon is packaged.
• The alternative is to provide an extra parameter (e.g., url4w) which does the same, but is specifically meant for Window users. A possible advantage is that this makes it easier to explain in the help file.
• Making it possible to share addons using pip, as suggested by @stevenpawley - I think this is a great suggestion, but I see it as an option that complements this request as it is tailored towards another type of user.
• Create a addon that offers the same functionality as g.extension, but with the option to point to other repositories or a local zipped addon file. This could perhaps serve as a temporary solution?

Additional context

As use cases:

• I am giving a course in which students use the v.in.gbif addon. They encountered a bug, I have a solution (#1248), but to be able to easily share the update, the update needs to be available on the official download site of Windows addons. That sometimes takes time, which is understandable, but inconvenient when running a course.
• Furthermore, I actually would prefer to be able to share bug fixes for further testing before making a final commit to the official repository. But to keep my students on board, it should be as easy as possible for them to install the patch.
• Another user case is sharing tailor made addons, e.g., for specific courses, that are not suitable to share through the official repository.

[nilason]

Is this really an issue for 8.4.1?
(As far as I understand Win addons server is working again).

[ecodiv]

Is this really an issue for 8.4.1? (As far as I understand Win addons server is working again).

Not an issue, but still a feature request. I still think it would be very useful to make it possible for Windows users to install addons from other sources.