Allow to Select Agent in Environment setting page

Problem

For Render Kubernetes resources in Environment inde... (#390769 - closed), users need to specify which agent to be used for fetching Kubernetes resources.

Proposal

1. User visits Project > Deployments > Environments. This is Environment index page.
2. User clicks an environment name, which navigates to Environment details page.
3. User clicks "Edit" button
4. User selects an available agent from dropdown list.

Prerequisites

To do this, we likely need to finish Show shared agents in deployment projects (#395498 - closed) for letting deployment projects fetch shared agents. Currently, this is not possible due to lacking read_cluster permission.

Permission

It follows update_environment permission. Developer+ is allowed to edit. If the environment is protected, users who are allowed to access can update the environment.

UI

Technical details

• We add cluster_agent_id FK column to the environments table.

# Existing table
environments:
  name:
  cluster_agent_id: FK to clusters_agents table.

• In the environment edit page, frontend shows a drop down to select an agent. The agent list can be fetched with userAccessAuthorizedAgents GraphQL query.
• Users choose one of the agent and click "Save" button. The form is submitted to GraphQL Environment:Update or Environment:Create mutation. backend persists the information into the foreign keys above.
• The specified agent (Types::Clusters::AgentType) is exposed in Types::EnvironmentType, something like:

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

• When the agent owner project unauthorizes the access from deployment projects, project.environment.clusterAgent will be null. In this case, users have to set a new authorized cluster.

A few notes:

• Currently frontend uses a haml with serializing the environment JSON, and requests to Controller's PUT endpoint. We do NOT extend the Controller endpoint in favor of GraphQL Environment:Update mutation.

Frontend implementation plan

• Refactor the edit_environment component to use GraphQl for getting environment data (!120642 (merged))
• Refactor the edit_environment component to use GraphQl mutation for storing data (!121091 (merged))
• Refactor the new_environment component to use GraphQl mutation for storing data (!121444 (merged))
• Add agents select to the environment_form (!121503 (merged))
• Retrieve agent information from the environment query (!121869 (merged))

Further iteration

• Allow to Select Agent in Environment Folder set... (#396408 - closed)

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