Provide all membership sources in API response

We require backend changes to support Add "Additional membership" field to member drawer (#502458).

We can add a memberships field to app/serializers/member_entity.rb that is all the other membership sources. This field will need to be explicitly requested as it will likely be expensive to compute.

memberships {
  [
    {
      accessLevel: <access-level>
      source: <source-details>,
    },
...more sources
}