[feature] Add 'Custom Expression' numeric format

[nyalldawson]

This numeric format allows users to craft a custom QGIS expression to format numbers. The expression can use the @value variable to retrieve the value to be formatted, and then use any standard QGIS expression function to format this as desired.

It can be used anywhere QgsNumericFormat is accepted, eg layout scalebars, elevation plots, layout tables, and color ramp legends

Sponsored by the Swiss QGIS User Group

[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.

Format available for credits

• Funded by NAME
• Funded by URL
• Funded by NAME URL
• Sponsored by NAME
• Sponsored by URL
• Sponsored by NAME URL

Thank you!

[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!

[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 e10283b)

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9216
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!