Expose associated agent and namespace in Environment GraphQL

Description

In Allow to Select Agent in Environment setting page (#396407 - closed), users can now associate an agent to an environment. This information should be able to be fetched via GraphQL, specifically Types::EnvironmentType.

Proposal

Extend Types::EnvironmentType to return associated agent.

Here is an example of GraphQL query:

{
  project(fullPath: "group/project") {
    id
    environment(name: "<environment-name>") {
      slug
      agent {
        id
        name
        project
      }
      agent_namespace
    }
  }
}