Juju 7058/fix pagination for entitlement

[SimoneDutto]

Description

In this PR we change the way we return pages of entries from OpenFGA.
Before, we returned a page containing only one entity kind, which caused confusion to users because if they asked for a pageSize 10 and we returned a pageSize 5 with a continuation token, they would assume there weren't more entries.

Now we try to fill the page of pageSize entries, independently of entity kind.

the second-page fetch of pageSize is to ensure that if we return a continuation token, there are at least one more entry in openfga.

Address #1419

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

Test instructions

Notes for code reviewers