backend(session): refactor access/SessionRoles and session/SessionRole into a unified schema

[r0liveir]

Context

Currently, we have a duplication issue on on these two enums, which might confuse future contributors

What to do

1. Unify both into an access.enums.CommitteeRole, which denotes the role of an user in that committe.
2. Refactor tests to have the new proposed structure when needed
3. Refactor session domain

Expected Behaviour

We expect an unified CommitteeRole, avoiding more future technical debt