MdeModulePkg: Add codes to support trailer parse in HttpLib.

MdeModulePkg: Add codes to support trailer parse in HttpLib.

In HttpLib, the Event BodyParseComplete should return to the
callback function when the whole message body has been parsed
including the trailer if it has.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18213 6f19259b-4bc3-4df7-8a09-765794883524