The SolrPrometheusExporter should detect whether to use a basicAuth secret

[HoustonPutman]

Currently in the PrometheusExporter spec, you must specify a basicAuthSecret if you have BasicAuth enabled on a solrCloud.

The operator should be able to detect if the SolrCloud is setup to use BasicAuth, and figure out the secret it is using if the use does not provide an explicit basicAuthSecret. This should work very similarly to how the zkConnection info and solrImage are also taken from the SolrCloud reference, if none are provided.

Docs: https://apache.github.io/solr-operator/docs/solr-prometheus-exporter/#prometheus-exporter-with-basic-auth