Show a color ramp legend for vector heatmap layers

[nyalldawson]

Instead of showing no legend for these layers, show the color ramp as a gradient bar.

Users can control the text for the min/max labels, the direction of the bar, and the font settings.

This also reworks the vector renderer API so that renderers have the flexibility to create any type of legend nodes, not just QgsSymbolLegendNodes. This may be useful for other renderers in future too.

Sponsored by Rubicon Concierge Real Estate Services

[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.

• The description will be harvested so please provide a "nearly-ready" text for the final changelog
• If possible, add a nice illustration of the feature. Only the first one in the description will be harvested (GIF accepted as well)
• If you can, it's better to give credits to your sponsor, see below for different formats.

You can edit the description.

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Thank you!

[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 d0d582b)

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9089
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!