[sensorthings] Add support for Multidatastreams

[nyalldawson]

This adds support for the Multidatastream entity type, as implemented in the SensorThings version 1.1 "MultiDatastream extension"

While the specification mandates that MultiDatastreams have an optional polygon geometry, I've encountered numerous servers which expose different geometry types for this entity or which return errors when attempting to read the geometries from MultiDatastreams. Accordingly we always expose an option to load MultiDatastreams as geometryless layers alongside the default option to load them as polygon layers, to handle a wider range of connections.

[qgis-bot]

@nyalldawson

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[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.

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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 8ba2f8c)

[nyalldawson]

Thanks @nirvn!

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#8984
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!