.pytool/Plugin: Add a plugin LicenseCheck

.pytool/Plugin: Add a plugin LicenseCheck

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2691
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2833
Add a plugin to check license conflict for new added
files in a patch. It will report out errors when meeting
files which are not contributed under BSD-2-Clause-Patent.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>