Add algorithm to manage GRASS addons

[AlisterH]

Description

See #53048

[AlisterH]

There is one issue - it would be good to get rid of the "region extent" advanced parameter. But if it is possible, I'm not sure how to do it.

[AlisterH]

I guess the algorithm to list addons would ideally write the list to a file as well.

[nicogodet]

I guess addons become an algorithm once installed. How do you manage this on QGIS side as it requires a description file to be added to processing provider ?

[AlisterH]

See #53048
The simplest option is for there to be a directory for user-installed description files somewhere in the QGIS profile folder. But something more complex may be wanted, to avoid having some of the same problems as the processing models folder.

[AlisterH]

Installing description files would be a separate operation, because people wanting to write description files will want to get the GRASS addon first.

[AlisterH]

I guess the algorithm to list addons would ideally write the list to a file as well.

Done

[AlisterH]

There is one issue - it would be good to get rid of the "region extent" advanced parameter. But if it is possible, I'm not sure how to do it.

I haven't sorted this out, but my last commit overrides processInputs, which is unnecessary for these algorithms and was producing error messages.

[nicogodet]

This is automatically added in

QGIS/python/plugins/grassprovider/Grass7Algorithm.py

Lines 252 to 258 in 5f98f5c

	param = QgsProcessingParameterExtent(
	self.GRASS_REGION_EXTENT_PARAMETER,
	self.tr('GRASS GIS 7 region extent'),
	optional=True
	)
	param.setFlags(param.flags() | QgsProcessingParameterDefinition.FlagAdvanced)
	self.params.append(param)

Not easily removable

[nyalldawson]

It might not be perfect yet, but we can always refine in follow ups...

[nyalldawson]

@AlisterH can you fix the failing flake test please?

[AlisterH]

Done.