SourceLevelDebugPkg: Use Pcd for the revision of transfer protocol

SourceLevelDebugPkg: Use Pcd for the revision of transfer protocol

V3 changes:
Add detailed description for the usage of the Pcd in package DEC file.
Update the comment for the introduced Pcd in module INF files.

V2 changes:
Instead of using a global variable, use a Pcd for transfer protocol
revision.

Previously, the revision of the debug agent transfer protocol is
reflected by a macro.

This commit introduces a Pcd to reflect the revision in order to avoid the
comparison of two macros, which will generate a constant result detected
by code checkers.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>