Add postHook operation workflow + use posthook to register a builder in Buildernet recipe

[ferranbt]

This PR adds two things:

• A new workflow to register post-hook operations that are executed after the recipes are ready.
• The use of the post-hook to register a new builder in Builderhub once it is ready.

To register a builder with the BuilderHub recipe run:

$ go run main.go cook buildernet --output ./output  --builder-config sample-config.yaml --builder-ip ...

If the builder-config is not set, the recipe will not register any builder. The config must be supplied in Yaml format, the recipe will overlay the info about the genesis file on the "genesis" key.