feat: Add policy to get sessions for a given subject

[ZohebShaikh]

1. Add enpoints to get all the sessions a subject is allowed to access.
2. The scopes that a subject can have depending on the audience claims
3. Add allow if all the tags are valid(visible) sessions for the subject

[tpoliaw]

This looks like it's very specific to tiled. Should we be using a separate application policy that imports the central version?

[ZohebShaikh]

Are you suggesting to move the policy to authz/policy/tiled from authz/policy/diamond/policy ?
Did you find any issue in the actual policy other than this ?

[tpoliaw]

I meant more having a separate bundle, something like this. For the policy itself, it looks ok but I might need a walk through to explain what it's doing - rego is baffling.

[ZohebShaikh]

Closing in favour of this #293