Expose NATS API for applications

[autodidaddict]

Create a NATS API that exposes (at least) the following functionality:

• Put an application spec (OAM model)
• Query an application spec
• Query list of applications
• Obtain application status
• Roll back/delete an application
• Query application event log

This issue also includes implementing the underlying persistence mechanism for deployments. At implementation time, we'll need to decide if we're going to use global/distributed ETS or use JetStream.