[sensorthings] Implement subset string support

[nyalldawson]

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.

[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[qgis-bot]

@nyalldawson

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[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 7acbb12)

[nirvn]

All good.

I'm curious, why did sensorthings protocol go for a custom filtering syntax?

[nyalldawson]

@nirvn

I'm curious, why did sensorthings protocol go for a custom filtering syntax?

No idea!