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>