ArmVirtPkg: Add Platform CI and configuration for Core CI

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2570



Add new Azure Pipeline definitions to build and run ArmVirtPkg with:
  * Ubuntu GCC5
Add PyTool based build of ArmVirtPkg
Add extdep for managing the iasl dependency
Add ArmVirtPkg.ci.yaml for Core CI
Add ReadMe.md for details and instructions

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

Reviewed-by: Shenglei Zhang <shenglei.zhang@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>