worker: use openapi spec and generated code

Write an openapi spec for the worker API and use `deepmap/oapi-codegen`
to generate scaffolding for the server-side using the `labstack/echo`
server.

Incidentally, echo by default returns the errors in the same format that
worker API always has:

    { "message": "..." }

The API itself is unchanged to make this change easier to understand. It
will be changed to better suit our needs in future commits.

internal/worker/api/api.go

@@ -0,0 +1,3 @@
+//go:generate go run github.com/deepmap/oapi-codegen/cmd/oapi-codegen -package=api -generate types,server -o api.gen.go openapi.yml
+
+package api