MdeModulePkg:DxeHttpLib: Update to DxeHttpLib API

MdeModulePkg:DxeHttpLib: Update to DxeHttpLib API

Rename and update the logic of HttpGenRequestString API provided
by DxeHttpLib. The RequestString size is returned as an argument.
The user is not expected to do a AsciiStrLen anymore, and is not
logical too, since request string can contain message body and
using AsciiStrLen on such a string can provide truncated lengths.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Nagaraj Hegde <nagaraj-p.hegde@hpe.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Samer El-Haj-Mahmoud <elhaj@hpe.com>