Request subset of attributes for LayerFeaturesModel, rework RelationReferenceFeaturesModel

[uclaros]

This PR adds a QgsAttributeList mAttributeList to LayerFeaturesModel, which is used to limit the attributes fetched when populating the models using QgsFeatureRequest::setSubsetOfAttributes.
By default all attributes will be fetched, but for example in MMFeaturesListPage we can now only request the attributes used for the feature title.
This improves performance when the layers have expensive virtual fields or potentially huge number of attributes.

The RelationReferenceFeaturesModel is also reworked so that it only populate a model with required attributes (relation key attributes and the ones used by display expression)

[github-actions]

Pull Request Test Coverage Report for Build 16913641553

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 143 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.1%) to 59.561%

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Files with Coverage Reduction	New Missed Lines	%
input/app/layerfeaturesmodel.h	1	0.0%
input/app/position/providers/simulatedpositionprovider.cpp	1	90.91%
input/app/featuresmodel.cpp	2	46.58%
input/app/inputhelp.cpp	8	35.0%
input/core/merginuserauth.cpp	9	68.24%
input/core/merginapi.cpp	13	75.13%
input/app/layerfeaturesmodel.cpp	19	80.49%
input/app/relationreferencefeaturesmodel.cpp	21	63.86%
input/app/inpututils.cpp	69	52.39%

Totals
Change from base Build 16909520962:	-0.1%
Covered Lines:	8323
Relevant Lines:	13974

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💛 - Coveralls

[tomasMizera]

Hi @RastoHu @IvaKuklica, here are the builds to test:

• 🍎 iOS: 25.11.824311
• 🤖 Android: 730251
• 🪟 Win: Exe from artefacts here

[IvaKuklica]

Regression testing for:
🍎 iOS: 25.11.824311
🤖 Android: [730251]
🪟 Win: Exe from artefacts here

STATUS: Completed
COMMENT: During the testing, some previously identified issues reappeared and tickets are already in place.