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/Makefile

@@ -0,0 +1,3 @@
+tag:
+	@git tag `grep -P '^\tversion = ' echo.go|cut -f2 -d'"'`
+	@git tag|grep -v ^v