[feature][expressions] Add "crs_ellipsoid" property to layer_property function and add layer_crs_ellipsoid variable

[agiudiceandrea]

Description

• Adds the layer_crs_ellipsoid variable to retrieve the ellipsoid acronym of the current layer CRS.
• Adds the "crs_ellipsoid" property to the layer_property function in order to retrieve the ellipsoid acronym of the layer CRS.

They are both useful for use e.g. with the bearing function.

[qgis-bot]

@agiudiceandrea
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]

@agiudiceandrea
A documentation ticket has been opened at qgis/QGIS-Documentation#8504
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!

[qgis-bot]

@agiudiceandrea

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!