Fixes #300

Solr supports using a different TLS certificate (keystore & truststore) for making client requests (to other Solr pods) than the server certificate. This PR enhances the TLS configuration options to support this additional client certificate.

I also cleaned up the Prometheus exporter TLS code to allow only using a truststore as the exporter doesn't need a keystore unless mTLS is enabled since the exporter only exposes HTTP endpoints. In other words, the exporter config should support users only providing a truststore and not require a keystore (for flexibility).

I also renamed mountedServerTLSDir to mountedTLSDir since I introduced a spec.SolrClientTLS to the SolrCloudSpec struct; this is a new struct added for v0.4.0, so no back-compat concerns with this rename. When I originally created mountedServerTLSDir, I was thinking I would have a mountedClientTLSDir as well for this feature but I think having an additional top-level SolrClientTLS on the SolrCloudSpec is better since we don't want to introduce ClientKeystore + ClientKeystorePassword + ClientTruststore ... for the non-mounted dir option. Also, mountedTLSDir works better for the Prometheus exporter CRD since it's only a TLS client and doesn't serve HTTPs traffic.

It probably doesn't make sense to review the "diff" for controllers/util/solr_tls_util.go and instead just go over the entire file since I had to make so many changes there to accommodate these changes.

Here's an example SolrCloud config to put these changes into context:

  solrTLS:
    clientAuth: Need
    checkPeerName: true
    verifyClientHostname: true
    restartOnTLSSecretUpdate: true
    keyStorePasswordSecret:
      name: pkcs12-password-secret
      key: password-key
    pkcs12Secret:
      name: dev-selfsigned-cert-tls
      key: keystore.p12
    trustStoreSecret:
      name: dev-selfsigned-cert-tls
      key: truststore.p12

  solrClientTLS:
    restartOnTLSSecretUpdate: true
    keyStorePasswordSecret:
      name: pkcs12-password-secret
      key: password-key
    pkcs12Secret:
      name: dev-selfsigned-client-cert-tls
      key: keystore.p12
    trustStoreSecret:
      name: dev-selfsigned-client-cert-tls
      key: truststore.p12