[server] Add WMS parameter value to request only maptip for HTML feature info response

[pathmapper]

This is a proposal to add a new value HTML_FI_ONLY_MAPTIP for the existing WMS vendor parameter WITH_MAPTIP.

If set, the feature info HTML response includes only the maptip, which gives full control over the HTML response using e.g. the built-in maptip editor which has a nice preview since #52420 and also got other improvements in #54764.

This aims to be fully backward compatible while closing #20601.

Tests included.

[github-actions]

🪟 Windows builds ready!

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(Built from commit e13d9aa)

[qgis-bot]

@pathmapper
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.
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Thank you!

[qgis-bot]

@pathmapper

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Thank you!

[pathmapper]

Any chance to get this in for 3.36?

[elpaso]

LGTM, just a few minor comments.

[github-actions]

@pathmapper
A documentation ticket has been opened at qgis/QGIS-Documentation#8794
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!