Add metadata setting to override widget wrapper used for a parameter

[alexbruy]

Description

While it is possible to add new widget wrappers for Processing parameters there is no way to override standard widget wrappers defined for each parameter type. This was possible using old API and added a lot of flexibility in different use cases.

This PR adds support for a new parameter metadata setting "widget_type" which allows to allows to override standard widget wrapper for a parameter.

param.setMetadata( {'widget_wrapper': { 'widget_type': 'my_widget' }} )

[alexbruy]

@nyalldawson as we discussed some time ago.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit ef1c9df)

[qgis-bot]

@alexbruy

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