ShellPkg/TftpDynamicCommand: Add one option for tftp command to specify windowsize.

ShellPkg/TftpDynamicCommand: Add one option for tftp command to specify windowsize.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=886

This patch is to define one new option for TFTP shell command to specify the
windowsize option as defined in RFC 7440. Valid range is between 1 and 64,
default value is 1.

Note that: RFC 7440 does not mention max window size value, but for the
stability reason, the value is limited to 64.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Carsey Jaben <jaben.carsey@intel.com>
Cc: Shao Ming <ming.shao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>