Add re-tracker Search API Index implementation for Strawberry Flavors

[DiegoPino]

What?

Strawberry Flavor data sources are not Content Entities, just virtual structures saved inside the permanent key-value storage. On re-tracking of an index we need to add our own implantation. Means

• Act on an event
• Read in batches SBFlavor collection items and validate if their parents (nodes and files) still exist. If not also delete. The later already happens automatically on any ADO change/file removal but I can see how tracker might not be updated 100% of the time bc of race conditions related to Drupals DB -read only committed - approach which implies sometimes a deletion might happen inside the same transaction as an untracking but the later is not aware of that (or the other way around)

[DiegoPino]

Fixed via 393da64. Not you should be able to re-track SBF. Only question (to be determined?) is if there could be any issues with non committed SBF to key value in a single Transaction? We will know