Add views relationship to group entities from og_membership entities

Problem/Motivation

OgMembershipViewsData exposed a relationship to users via
uid but never offered a relationship from entity_id back to
the group entities themselves. This meant Views could not join an OG membership to its
parent group (node, taxonomy term, etc.), so group-level fields were unavailable.

Proposed resolution

• Register one relationship per enabled OG group entity type by looping through
  GroupTypeManager::getGroupMap().
• Each relationship joins og_membership.entity_id to the primary key of the
  group entity’s data table and is limited with
  extra => entity_type = <type_id>.
• Expose an optional filter on entity_bundle so view builders can restrict
  results to specific bundles when needed.