Ensure page token is for the same tree

The page token returned by GetTreeEntries does not include the tree oid and can page across two different trees leading to incorrect results. In order to resolve this we can include the tree oid in the page token and use it to ensure the page token is always used to access a given tree only.

Reopened the MR has the previous fix caused this incident - gitlab-com/gl-infra/production#18286 (closed)

As the changes would be customer facing so added a feature flag - treeoid_pagination

Changelog: fixed

Closes #4556 (closed)