feat: add Lookup API to lookup the relations a subject has to one or more objects

[jon-whit]

Given one or more relations and a subject, the goal of the Lookup API is to determine the object(s) for which the user has the given relation(s). The query should resolve by starting at the subject node in the Graph of Relations and doing a reverse traversal until all of the object nodes of the graph have been reached from the initial subject node.

[kevinz]

Sounds like similar to https://docs.authzed.com/v0/api/#aclservicelookup, right?

[jon-whit]

Sounds like similar to https://docs.authzed.com/v0/api/#aclservicelookup, right?

Yep, same idea. The goal is to be able to lookup a subjects specific relations to one or more objects in one or more namespaces. This differs from the Expand API which returns all subjects and subject sets having a certain relation to an object.

The lookup API will be better suited for front-end UI component filtering and authorization filtering, just to two name two examples.