make XML Query Parser support extensible-via-configuration

SOLR-839 added XML QueryParser support (deftype=xmlparser) and this ticket here proposes to make that support extensible-via-configuration.

Objective:

• To support use of custom query builders.
• To support use of custom query builders without a corresponding custom XmlQParser plugin class.

Illustration:

• solrconfig.xml snippet to configure use of the custom builders

  <queryParser name="testxmlparser" class="XmlQParserPlugin">
    <str name="HelloQuery">org.apache.solr.search.HelloQueryBuilder</str>
    <str name="GoodbyeQuery">org.apache.solr.search.GoodbyeQueryBuilder</str>
  </queryParser>
  

• HelloQueryBuilder and GoodbyeQueryBuilder both extend the new abstract SolrQueryBuilder class.

  + public abstract class SolrQueryBuilder implements QueryBuilder {
  +   protected final SolrQueryRequest req;
  +   protected final QueryBuilder queryFactory;
  +   public SolrQueryBuilder(String defaultField, Analyzer analyzer,
  +       SolrQueryRequest req, QueryBuilder queryFactory) {
  +     this.req = req;
  +     this.queryFactory = queryFactory;
  +   }
  + }