New template sharing option for coder_apps

[stirby]

Some users want to share coder_apps with a specific group or users, rather than all authenticated users on a deployment. We should support this with a new "template" share attribute for coder_apps.

This would open the app to any users who are granted access to the template. While this is a slightly convoluted method of sharing with groups, it best aligns with our current RBAC.

resource "coder_app" "vim" {
  agent_id     = coder_agent.dev.id
  slug         = "vim"
  ...
  share        = "template"  
}

Related: coder/coder#13361