ShellPkg/tftp: Convert from NULL class library to Dynamic Command

UEFI Shell spec defines Shell Dynamic Command protocol which is just
for the purpose to extend internal command.
So tftp command is changed from NULL class library to be a driver
producing DynamicCommand protocol.

The guideline is:
1. Only use NULL class library for Shell spec defined commands.
2. New commands can be provided as not only a standalone application
   but also a dynamic command. So it can be used either as an
   internal command, but also as a standalone application.

TftpApp.inf is to provide a standalone application.
TftpDynamicCommand.inf is to provide a standalone driver producing
Dynamic Command protocol.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>