[sensorthings] Implement subset string support (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request qgis/QGIS#56670 Author: @nyalldawson QGIS version: 3.38

[sensorthings] Implement subset string support

PR Description:

Allows setting custom filters on the layer, using the SensorThings filter syntax.

I've included a custom layer filter GUI for SensorThings layer filters, making it easier for users to craft valid filters using the SensorThings filter language.

Commits tagged with [need-docs] or [FEATURE]