fix: allow percent sign in getFeature filter

[steff-o]

Fixes #1848 by moving the wfs part of getFeture() to WfsSource class as it already urlecodes arguments correctly.
Also fixes #1523 when strategy = 'all' as that wasn't fixed in #1579
Probably also solves at least part of #1534, but on the other hand, it makes #1535 and #1536 almost impossible to merge.

As a bonus some redundant code from getFeature is removed and hopefully it will be easier to implement filters for QGIS and other servers and support for different WFS versions etc by keeping all code in one place.

I have tried to make it as backwards compatible as possible, but there is a small risk that if someone utilized or tried to compensate for the weaknesses (e.g. no coordinate transform) in a plugin or external code that may break. To my knowledge getFeature is used in search (internally in Origo) and from the MultiSelect plugin, which should be unaffected. The things that are deliberately not entirely backwards compatible are:

• The WFS version in getFeature was 1.0.0. This implementation will use the same version as WfsSource, which right now is hardcoded as 1.1.0
• The request will follow the method (GET, POST) that the layer is configured with. Old implementation of getFeature was always GET.

None of those should pose any problem unless the layer is configured only to be used by getFeature and you have a special 1.0.0 server for that that or it does not support post requests but you have configured the layer as 'POST' anyway.

[jokd]

Seems to work as intended so LGTM

[filleg]

Works well with QGIS server, nice cleanup!

Except the filters with feature id, but that's a separate issue as you mentioned.

#1535 is no longer necessary since QGIS server 3.28, which added support for different projections with GeoJSON.