Implement admin logic

[hug-dev]

In the config file

We add under the [authenticator] section the admins field:

admins = [ { name = "admin_1" }, { name = "admin_2" } ]

The type of name is authenticator-dependant but would map to application names that are considered as admin for that authenticator: a SPIFFE ID, a UID, a string (for direct auth).

In the service builder

When building the authenticator structure (implementing Authenticate), we store the information of the admins application name in the authenticator structure.

In the front-end handler

The front-end handler calls the authenticate method on the authenticator to then dispatch the request with the good application name (or none). Things here stay the same but the ApplicationName structure would be enhanced with a boolean field indicating if it is an admin application name or not (is_admin). It's the authenticator during the authenticate call that would set that boolean or not if the application name is in the list of the admin ones.

In the back-end handler

Before calling the appropriate Provide method, for those that require admin privilege, is_admin is called to check if the application has privilege or not. If not a AdminOperation error should be returned (see parallaxsecond/parsec-interface-rs#93).

Testing

Will be done in another issue, in #309