Allow `--custom` when adding transformers
I discovered in my dogfooding project (https://gitlab.com/meltano/meta/-/issues/234), that adding --custom
transformers is explicitly disallowed as of now and this is problematic for the path we are going with #3298 (closed).
We should allow transformer
plugins to be defined via --custom
as we do today for extractor
plugins and other types.
Edited by AJ Steers