Swagger docs for the REST API

[Maelkum]

This PR adds swagger documentation for our REST API.

The OpenAPI specification can be found in /api/b7s-swagger.yaml.

To generate code from this we use https://github.com/deepmap/oapi-codegen. I've created a Makefile that supports creating three distinct things - models, server and client code, in three separate files - models.gen.go, server.gen.go and client.gen.go. I think this makes things more easily manageable and less noisy.

I opted to keep using our existing models (the ones we use internally to handle execution requests and all of that) as part of the public API. The downside is - if we start changing these, the public interface changes. Alternative is to create separate models for the REST API that can be more stable; however we'd have to duplicate everything we use internally, and our models are relatively deep. I didn't see much value in that at this point.

By default, any non-required parameter in the schema is generated as a pointer. E.g. if field is textual and not required, it will be rendered as a Field *string go type, as opposed to a regular string. IMO this makes Go codebase overly verbose and doesn't bring in much value (in most cases "" is as good as nil when determining what to do). Unfortunately to avoid the pointers, I had to add a x-go-type-skip-optional-pointer flag for each field. Ugly when reading swagger YAML, but doesn't show up when rendered and Go code is better.