Generic provisioners

[begelundmuller]

This PR refactors the admin service to support generic provisioners. This paves the way for not only provisioning managed runtimes, but also managed ClickHouse clusters and more.

Main changes:

• Introduces a generic Provisioner interface in admin/provisioner/provisioner.go
• The provisioner interface centers around creating/updating/checking/removing "resources":
  • Resources are managed per deployment
  • Resources have state which the provisioner can update
  • Resources are tracked in a new database table provisioner_resources
• Refactors the validate_deployments and check_provisioner_capacity jobs to use health check functions on the provisioner interface

Potential future work:

• We now create a deployment with status=pending before provisioning a runtime for it. We should move completion of the deployment to a retryable background job.
• Support updating the slots assigned to an existing deployment when the provisioner allows it.
• The provisioner interface's Provision and CheckResource functions are very similar, particularly because Provision is also used for updates. We should consider merging these three functions into one Reconcile function.
• Support the provisioner changing its config after the initial provisioning (by having some kind of reconcile for instance config).