Make nexus::system_orchestrator::Context thread safe

[Yadunund]

We often pass the Context to BT nodes but the operations are not thread safe when we run multiple jobs in parallel. We could update workcell_task_assignments in the system orchestrator callbacks while BT nodes might be iterating over the same.

We should move all data members to private scope and add get/setters that lock a mutex before accessing these data members.