Add internal Kubernetes Agent API

Creates an experimental, internal Kubernetes Agent API. Part of &3329

Feature flag: kubernetes_agent_internal_api

Also creates the necessary tables and models to support the API

• create agents table
• create agent_tokens table
• Use agent token to authenticate to this new API (Authorization: Bearer TOKEN)
• If token is invalid, respond with 403 Forbidden
• [-] If token is has been revoked, respond with 403 Forbidden. Will follow-up in new issue

In response we also need:

• agent_id
• agent_name
• project_id (can be added later, not urgent)
• [-] cluster_id (can be added later, not urgent) Will follow-up in new issue

Related issue: gitlab-org/cluster-integration/gitlab-agent#1 (moved)

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.

Notes for anyone who wants to try the branch:

• gdk update and switch to the branch

• run bundle exec rails db:migrate RAILS_ENV=development

• In Rails console (bundle exec rails console):

  Feature.enable(:kubernetes_agent_internal_api)
  
  p = Project.find(<id>)
  agent = Clusters::Agent.create!(project: p, name: "my-agent")
  token = Clusters::AgentToken.create!(agent: agent)
  puts token.token # this is the token for the agent