This adds `auth` option and deprecates `cookie` which has a confusing
name. It's effectively renaming but we pretend it is not to avoid people
blindly changing the name without second thought.
Closes#349 (a new tracking issue will be created)
This change takes several steps towards avoiding footguns when
configuring `electrs`:
* Moves config_spec.toml out of sight of non-developers
* Provides a configuration example
* Mentions the example in the docs