ArmPkg/ArmMonitorLib: Definition for ArmMonitorLib library class

The ArmMonitorLib provides an abstract interface to issue
an HyperVisor Call (HVC) or System Monitor Call (SMC) depending
on the default conduit.
The PcdMonitorConduitHvc PCD allows to select the default conduit.

The new library relies on the ArmHvcLib and ArmSmcLib libraries.
A Null instance of these libraries can be used for the unused conduit.

Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>