Enable multiple authenticators to work simultaneously

[hug-dev]

Currently only one authenticator is allowed to work in Parsec.
The reason is that the OnDiskKeyInfoManager stores the KeyTriple to KeyInfo mappings as a directory structure:

    /// mappings_dir_path/
    /// |---app1/
    /// |   |---provider1/
    /// |   |   |---key1
    /// |   |   |---key2
    /// |   |   |   ...
    /// |   |   |---keyP
    /// |   |---provider2/
    /// |   |   ...
    /// |   |---providerM/
    /// |---app2/
    /// |   ...
    /// |---appN/

If we were to add one more layer for AuthType, and then transform the KeyTriple into a KeyFourtuple, old mappings will not be compatible with new ones directly.

That task is about making the necessary changes in Parsec for it to work with multiple authenticators securely. There are changes needed:

• in the ManageKeyInfo trait and the key_info_managers modules
• in the Provide trait and implementations to support passing, along the ApplicationName, the AuthType as well

One thing that could also be done is implement a new KeyInfo Manager that supports multiple authenticators and that supports it from start! That way the restriction will only apply to the old OnDiskKeyInfoManager.

[hug-dev]

To be looked at in parallaxsecond/parsec-book#83