Auth mode inferred from the Authorization header

In HIVE-27352 we added support for multiple authentication modes and this change introduced the "auth" http header, 
which broke compatibility with Impala in the following case: The impala-shell client tool is able to connect to HS2 with the authentication mode specified first in the auth mode list, but the other auth options do not work because the impala-shell does not send an "auth" header to HS2. In a discussion with impala devs, it turned out that impala does not need a similar header to identify the authentication mode, because in case of http transport, the content of the Authorization header can be used to infer the auth mode. This improvement aims at avoiding the use of the "auth" header and thus allows us to connect to HS2 via http protocol even if multiple authentication modes are enabled and the client does not use the "auth" header (e.g. impala-shell or older jdbc driver builds).