Annotated corpus map: add output option with keywords (cluster labels) per cluster

**Is your feature request related to a problem? Please describe.**
After generating clusters with Annotated Corpus Map, I'd like to create a table describing the clusters, e.g. average year of publication, most frequently occurring publisher, etc., but also the words that are typical for the documents in the cluster (cluster labels in Annotated Corpus Map). I can do most of this with Group By, but there is no way to extract the characteristic words per directly.

**Describe the solution you'd like**
Add an output option for Annotated Corpus Map with keywords (cluster labels) per cluster - preferably with a user-definable maximum (not just the 5 that are produced when cranking up the 'Cluster labels' slider).

**Describe alternatives you've considered**
Let's say I have 10 clusters, I could do Select Rows to select a cluster, then Extract Keywords per cluster. I have to do this 10x in parallel, then Concatenate and Group By Source ID to get an overview of the words per cluster which I could merge with the other grouped data per cluster. But this gives me slightly different keywords, and the replication needed to treat each cluster in parallel makes this a cumbersome workaround - especially because it hardly allows me to vary the number of clusters (which necessitates adding/removing parallel branches of Select Rows -> Extract Keywords)

Start with the Annotated Corpus Map workflow and its existing Cluster labels slider and output behavior. Determine how cluster-level results are currently exposed, then define the keyword output and user-configurable maximum; done means each cluster can provide its characteristic words alongside the requested cluster information.

Written by the indexing model from the issue text.