NetworkPkg: Add PCD to enable the HTTP connections switch

NetworkPkg: Add PCD to enable the HTTP connections switch

v3:
* Correct the commits grammar

v2:
* Rename the PCD to PcdAllowHttpConnections.
* Refine the PCD descriptions.

If the value of PcdAllowHttpConnections is TRUE, HTTP connections are
allowed. Both the "https://" and "http://" URI schemes are permitted.
Otherwise, HTTP connections are denied. Only the "https://" URI scheme
is permitted.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>