!!!FEATURE: Support asset collection count

[sorenmalling]

Upgrade instructions

The SupportsCollectionInterface have a new method countByAssetCollection. If your AssetProxy implements that interface, you must include that method in your implementation

Review instructions

• Checkout this PR
• Add images to a certain collection
• Add images from your AssetProxy to a collection
• See the number adjust accordingly

Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked wit !!! and have upgrade-instructions

[sorenmalling]

For reference, this is a extension of the functionality created for tags

#3451

[bwaidelich]

Isn't the master branch supposed to be readonly by now (so this should target 8.1 instead) ? – well, or probably 9.0 even since this is a breaking change?

[sorenmalling]

Isn't the master branch supposed to be readonly by now (so this should target 8.1 instead) ? – well, or probably 9.0 even since this is a breaking change?

@bwaidelich Let me know, and I'll change it :)

[bwaidelich]

@sorenmalling According to #3713 the master branch will be renamed to 8.1. So if this is a breaking change it should be against 9.0

[Sebobo]

I fear this could get slow without further optimisation in case of many asset collections.
A native query that does all of this in one query would better. Similar to what I did in the Media.UI Flowpack/media-ui@7bf8e96