Add "Anonymous" credentials for servers allowing access without credentials

[Tropid]

This allows to connect to a server without specifying credentials (if the server supports it):

Jira::new("http://localhost:8080", Credentials::Anonymous)

It adds a new variant "Anonymus" to the Credentials enum.

This may break clients if they match on this enum exhaustively. It may also be a good idea to add the new non_exhaustive attribute to this enum to allow future additions without breaking client code.