[raster] New single color renderer

[nirvn]

Description

This PR implements a single color renderer for raster layers. This allows for a super simple and friendly way to render a raster using a single color:

Screencast.from.2024-04-08.12-47-22.mp4

[nyalldawson]

Nice work!

I think we'll also need to expose methods to set the desired input band here, and add a band selection combo to the renderer's widget.

[nirvn]

@nyalldawson , yep, it occurred to me we still need a band selection. It'll come today.

[qgis-bot]

@nirvn
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]

@nirvn
A documentation ticket has been opened at qgis/QGIS-Documentation#9000
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!