Cluster Agent create mutation for GraphQl

What does this MR do?

Related to issue: gitlab-org/cluster-integration/gitlab-agent#6 (closed)

We recently created an Agent Model and Token Model. MR: !33228 (merged)

Now that models have been created, we need to make GraphQl calls to access agents and tokens

Per another discussion, this feature will only be available for premium plans

This MR will:

• Add GraphQl Clusters::Agent create mutation
• Add GraphQl Clusters::Agent type
• Add Clusters::Agents::CreateService
• Add Clusters::AgentPolicy (for authorize :admin_cluster in Clusters::AgentType)
• Added associated tests
• Updated associated GraphQl documents

Future MRs will:

• Add GraphQl Clusters::Agent delete mutation
• Add GraphQl Clusters::Agent query
• Add GraphQl Clusters::Token create mutation
• Add GraphQl Clusters::Token delete mutation
• Add GraphQl Clusters::Token query

Screenshots

Create Token create

GraphQl Error Response: error

mutation {
 createClusterAgent(input: { projectPath: <project-path>, name: <cluster-agent-name> }) {
  clusterAgent {
    id 
    name
  }
  errors
 } 
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team