Add merge policies

[JuhoErvasti]

Fixes #59494.

Similarly to already existing Split and Duplicate "policies" this PR implements Merge Policies, which determine the initial values in the Merge Features dialog. The PR includes the following merge policies:

From the existing Qgis::FieldDomainMergePolicy enum:

• Sum: numeric fields only
• Geometry Weighted: numeric fields only, uses weighted average by geometry
• Default Value: uses the default value set in QGIS

New policies:

• Unset Field: similarly to Split & Duplicate policies clears the field and lets the data provider populate any default value if one exists. If it doesn't the first feature is used (this is the behavior which is currently implemented)
• Largest Geometry: use the value from the feature with the biggest geometry (length/area/number of parts for multipoints)
• Minimum Value: numeric fields only
• Maximum Value: numeric fields only
• Skip Attribute: sets the initial state to "Skip", if not modified manually value will be NULL

Initial merge dialog before PR:

Initial merge dialog after PR:

Configuring the policies:

[github-actions]

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