Rewrite Mock to handle known authorities

[mbklein]

Description

Authoritex.Mock has no way to know whether a given unregistered URI is from an unknown authority or a missing ID from a known authority.

Current Implementation

Per #42, Authoritex.Mock.search("missing_id_authority:whatever") will return {:error, 404}, while any other unregistered URI will return {:error, :unknown_authority}

Proposed Implementation

• Update Authoritex.Mock.set_data() to include a list of known authority prefixes along with the map of IDs and labels
• Update Authoritex.Mock.fetch() to check the given URI against the list of registered authority prefixes (using String.starts_with? is sufficient)
  • If at least one authority prefix matches and the ID is registered, return the result as usual
  • If at least one authority prefix matches but the ID isn't found, the result is {:error, 404}
  • If no authority prefix matches even if the ID is found, the result is {:error, :unknown_authority}