Add last_used_at field to cluster Agent token

Scope

Provide the information on GraphQL for

• the token last_used_at as part of the token list
• an agent level max(last_used_at) computed in the backend

We can make the translation from last_used_at to status in JS-land.

Definition of Done

See discussion from issue: #321885 (closed)

• Add last_used_at field the to the rails database and Clusters::AgentToken model
• Build a process (similar to the Ci::Runner#heartbeat process) where last_used_at is updated during each kas interaction
• Update GraphQL: add field last_used_at
• Add index for last_used_at since GraphQl should sort tokens by last_used_at instead of created_at
• Update GraphQl sorting
• Add last_used_at field to UI token table