Support uploading configsets through the operator

**Is your feature request related to a problem? Please describe.**
I'd like to be able to apply a configset to a solr cluster managed through the operator.

**Describe the solution you'd like**
I could write a yaml file to define a configset.

**Describe alternatives you've considered**
Currently the only alternatives are to either build the image with the configsets or to upload one through the API before creating the configsets.

Start by reading the operator's CRD definitions and the existing configset upload/API handling. Define how a YAML-declared configset should be represented, applied to the managed Solr cluster, and tested; the issue does not name specific files or tests, so the scope needs maintainer clarification first.

Written by the indexing model from the issue text.