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.

vendor/github.com/labstack/echo/v4/log.go

@@ -0,0 +1,41 @@
+package echo
+
+import (
+	"io"
+
+	"github.com/labstack/gommon/log"
+)
+
+type (
+	// Logger defines the logging interface.
+	Logger interface {
+		Output() io.Writer
+		SetOutput(w io.Writer)
+		Prefix() string
+		SetPrefix(p string)
+		Level() log.Lvl
+		SetLevel(v log.Lvl)
+		SetHeader(h string)
+		Print(i ...interface{})
+		Printf(format string, args ...interface{})
+		Printj(j log.JSON)
+		Debug(i ...interface{})
+		Debugf(format string, args ...interface{})
+		Debugj(j log.JSON)
+		Info(i ...interface{})
+		Infof(format string, args ...interface{})
+		Infoj(j log.JSON)
+		Warn(i ...interface{})
+		Warnf(format string, args ...interface{})
+		Warnj(j log.JSON)
+		Error(i ...interface{})
+		Errorf(format string, args ...interface{})
+		Errorj(j log.JSON)
+		Fatal(i ...interface{})
+		Fatalj(j log.JSON)
+		Fatalf(format string, args ...interface{})
+		Panic(i ...interface{})
+		Panicj(j log.JSON)
+		Panicf(format string, args ...interface{})
+	}
+)