[feature] Allow end point marker symbols for balloon callout style

[nyalldawson]

Allows rendering a marker symbol below the endpoint of the balloon callout. Designed to allow balloon callouts to reproduce the same visual appearance as the older annotation framework items.

[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.

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[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 e889da1)

[nyalldawson]

I'll merge this later today if there's no objections before then

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9201
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!