The SolrPrometheusExporter should detect whether to use a basicAuth secret

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

Start with the PrometheusExporter spec and the SolrCloud reference behavior for zkConnection and solrImage, then read the linked BasicAuth documentation. Done means the exporter can infer BasicAuth and its secret from the referenced SolrCloud when basicAuthSecret is omitted, while still honoring an explicit value.

Written by the indexing model from the issue text.