MdeModulePkg/CapsuleApp: Add Fmp->GetImage() support.

MdeModulePkg/CapsuleApp: Add Fmp->GetImage() support.

We add Fmp->GetImage() support in CapsuleApp. So that user may call
Fmp->GetImage() in UEFI shell environment.
This is useful to do unit test for FMP which supports GetImage(),
or user wants to get current image, such as Microcode.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>