Add created_by_id to cluster_agents and cluster_agent_tokens

Spun out of this discussion !48320 (comment 466021421)

Proposal

1. Add colums created_by_id to both cluster_agents and cluster_agent_tokens.
2. Record this from current_user in Clusters::AgentTokens::CreateService

Tasks to complete

• Add column created_by_user_id to cluster_agents with GraphQL updates
• Add column created_by_user_id to cluster_agent_tokens with GraphQL updates