[cfid-433] Change authorizations for GET /Groups/{id}.

A new default group for users: scim.me enables users to perform
 1. GET operation on groups where they are listed as 'readers'
 2. PUT operation on groups where they are listed as 'writers'

Fixes [#41495249]. The same default group would eventually enable users to
read/write their own user objects. (Future task)

Change-Id: If12dd74c1afba26baa86de3fc462c6f79b72b50e