MdeModulePkg/DxeCore: set ImageContext Handle and ImageRead() fields

MdeModulePkg/DxeCore: set ImageContext Handle and ImageRead() fields

Set the ImageContext.Handle and ImageContext.ImageRead() fields so that
PeCoffLoaderRelocateImageExtraAction() can invoke PeCoffLoaderGetImageInfo
or PeCoffLoaderGetPeHeader if desired to obtain additional metadata.

We will use this to create a PeCoffLoaderRelocateImageExtraAction()
implementation that applies boot time strict mapping permissions to
PE/COFF modules.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Star Zeng <star.zeng@intel.com>