crimson: add AuthService

crimson: add AuthService

AuthService is introduced to allow Dispatchers to access to
authorizers, even if it is not chained with a Dispatcher implementing
Dispatcher::ms_get_authorizer(). in this case, we need to grant access to
Heartbeat class. it has its own messengers dedicated for heartbeat
traffic. it's mon::Client which provides the facilities of authorization
via Dispatcher interface.

we could just cast mon::Client to ceph::common::Dispatch for accessing
Dispatcher::ms_get_authorizer(), but i want to make this explicit using
AuthService. as the consumers of Dispatch inteface is messenger and
ChainedDispatcher not the domain specific classes.

in future, we need to either implement Auth{Client,Server} or adapt to
this machinery for msgr V2.

Signed-off-by: Kefu Chai <kchai@redhat.com>