Add vertical CRS selection widget to vector layer properties

[nyalldawson]

Follows the same behavior as the project setting, i.e.:

If the layer has a 3d crs set, then the widget is disabled with an explanatory note:

If the layer has a 2d crs set, then the user can select the appropriate vertical crs:

There's also a explanatory note detailing where vertical CRS are respected, along with a warning:

[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 2e59231)

[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

This pull request has been tagged for the changelog.

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Thank you!

[nyalldawson]

This is ready for merge

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9187
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!