cell type annotation

### Description of feature Would it be possible to match `cellranger annotate` [(see here)](https://www.10xgenomics.com/support/software/cell-ranger/latest/analysis/running-pipelines/cr-cell-annotation-pipeline)? I am aware that `nf-core/scdownstream` is under development and will include cell type annotation. But since they serve different purposes, including this step in both pipelines might be justified. I'm sure the topic has been thoroughly discussed in the past, I couldn't find a record of it though.