Allow new custom field entities to be added to reports
Now that we allow custom fields on almost any entity, it makes sense that we should allow CiviReport to make use of these. E.g. contribution reports should allow the use of FinancialType custom fields; relationship reports should allow RelationshipType custom fields, etc.
Ideally, this should be possible by adding the entity to the report's $_customGroupExtends array. However, this fails because CRM_Report_Form::customDataFrom() calls CRM_Core_BAO_CustomQuery::$extendsMap, a hard-coded list of tables which various entities extend.
I propose replacing calls to CRM_Core_BAO_CustomQuery::$extendsMap with a new method CRM_Core_BAO_CustomQuery::getExtendsMap(), which will also pull in any entities defined in civicrm_option_value.
I'm happy to do this work, I would like concept approval first though.