Offer default possibility to use an ES PageProvider for Admin NXQL search

The out-of-the-box nxql_search PageProvider is not using ES, which can lead real environments to experience MongoSocketReadTimeoutException when using the WebUI Admin/NXQL Search screen.

It is possible to modify that either by adding the PP to the nuxeo.conf property:

elasticsearch.override.pageproviders

Or by using directly a replacer contribution for it:

  <extension point="replacers" target="org.nuxeo.ecm.platform.query.api.PageProviderService">

    <replacer enabled="true" withClass="org.nuxeo.elasticsearch.provider.ElasticSearchNxqlPageProvider">
      <provider>nxql_search</provider>
    </replacer>

  </extension> 

It may be interesting to either make this PageProvider relies on ES by default, or have a configurable way out-of-the-box to allow users to switch from one to another.