Add a minimal data provider for plugin layers

[manisandro]

Throughout the codebase, there is a general assumption that layer->dataProvider() is not null. This wasn't the case for plugin layers.

I don't think there is actually a real use for data providers in the case of plugin layers, so such a minimal, non-exposed implementation should suffice.

[wonder-sk]

Just curious, where do you get problems in QGIS code due to data provider being null? In Crayfish plugin layer there is no data provider assigned and I haven't encountered any issues coming out of that...

[manisandro]

Just grep throughout the codebase for dataProvider()-> and you'll see many cases where the code operates on QgsMapLayer types without checking whether dataProvider() is null or not.