This refactoring aims to ease testability and clean up a lot of soft touchy
typings in test code.
This is the first step of this refactoring introducing the concept and
implementing missing interfaces and stubs. At the end of the day,
ServerVariablesHandler should completely disappear and every variable should
be injected in the endpoint handler builder itself.
One can now customize the default authentication method for all sub-domains,
i.e., either 'two_factor' or 'basic_auth' and define specific authentication
method per sub-domain.
For example, one can specify that every sub-domain must be authenticated with
two factor except one sub-domain that must be authenticated with basic auth.
Previously, logs were not very friendly and it was hard to track
a request because of the lack of request ID.
Now every log message comes with a header containing: method, path
request ID, session ID, IP of the user, date.
Moreover, the configurations displayed in the logs have their secrets
hidden from this commit.
