Support multiple relations when calling Read()

[aaguiarz]

The Read API supports reading tuples for a combination of a user_id, object_id/object_type and relation.

When listing explicit permissions for an object, you can call read in two ways:

• Specifying just an object_id, which will return all the tuples for all users and relations for the object_id.
• Specifying an object_id + relation, which will return tuples for all users, for a specific relation for the object_id.

In some scenarios you could want to read the tuples for an object_id and a set of relations (e.g. 'viewer' and 'writer').

Possible solutions are:

• The Read API can be extended to support an array of relations
• The SDKs can receive an array of relations and make multiple calls in parallel.