Add layer.params.default_fields array

[simon-leech]

This PR introduces the layer.params.default_fields[] option.
This is useful to add additional fields to the layer.params.fields that are not coming from the style (theme or labels).
This allows flexibility when working with bespoke plugins that required multiple feature fields to style on.

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[dbauszus-glx]

layer.params.default_fields should also be added to mvt feature loads.

layer.params.default_fields must not be spread. otherwise crunch on undefined. the array is flattened anyways. without the spread the default_fields will work as an array or a single string assignment.

layer.params.default_fields must be defined first. Because the theme may add n fields to the array. The setSource method receives features as wkt and the location of the desired property may change with different themes if not defined first.