XProfile: Introduce BP_XProfile_Meta_Query
class.
This class extends WP_Meta_Query
specifically for the Extended Profile component's shared metadata database table, which is unique in that it is used for metadata for field-groups, fields & field-options, and field-data for users.
The functionality of this class is not used in core yet, but will be used as part of #6347. Committing this in early to make iterating and testing easier during the 2.3 cycle.