Ability to set custom hostname for SolrCloud

Hello,

Much like the `addressability.external.domainName` value in the SolrCloud resource, it would be helpful to have something like `addressability.external.hostName` to specify a particular hostname for external accessibility. It appears now that the hostname is always set to `--solrcloud` (https://github.com/apache/solr-operator/blob/main/api/v1beta1/solrcloud_types.go#L1339) which is a bit redundant for my team's implementation of one SolrCloud per namespace (we end up with something like `solralpha-solralpha-solrcloud.domain.com`).

Thanks for your consideration!

Start in api/v1beta1/solrcloud_types.go around line 1339 and trace how addressability.external.domainName produces the external hostname for a SolrCloud. Check the related resource-generation entry points and existing tests, then confirm that a configured external hostName is honored without breaking the current default; done means the custom hostname is reflected in the generated external address.

Written by the indexing model from the issue text.