MdeModulePkg/BrotliCustomDecompressLib: Make brotli a submodule
    
    Use submodule way to access brotli in MdeModulePkg based on
    brotli version 666c3280cc11dc433c303d79a83d4ffbdd12cc8d.
    The newly added BrotliDecUefiSupport.h/.c are used by directory
    'brotli'.
    REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2559
    
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Jian J Wang <jian.j.wang@intel.com>
    Cc: Hao A Wu <hao.a.wu@intel.com>
    Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>