Allow to contribute RenditionDefinitionProviders

Only static rendition definitions can be contributed to the Rendition Service.

We need a way to have dynamic rendition definitions based on a document (for instance, for Video or Picture documents, expose what is stored in the schema as renditions).

Add a new extension point renditionDefinitionProviders to contribute class extending RenditionDefinitionProvider.

RenditionDefinitionProvider exposes only one method:

List<RenditionDefinition> getRenditionDefinitions(DocumentModel doc);

Rendition definitions and rendition definition providers contributions should be able to be filtered through standard filters.

Sample contribution:

<extension target="org.nuxeo.ecm.platform.rendition.service.RenditionService"
  point="renditionDefinitionProviders">

  <renditionDefinitionProvider name="dummyRenditionDefinitionProviderForNote"
    class="org.nuxeo.ecm.platform.rendition.service.DummyRenditionDefinitionProvider">
    <filters>
      <filter-id>onlyForNote</filter-id>
    </filters>
  </renditionDefinitionProvider>    

</extension>