hs: Encode flow control value in the descriptor

This simply adds the "flow-control" line, as detailed in prop324, to the
descriptor. No decoding is done at this commit.

Part of #40506

src/feature/dirparse/parsecommon.h

@@ -172,6 +172,7 @@ typedef enum {
   R3_DESC_AUTH_KEY,
   R3_DESC_AUTH_CLIENT,
   R3_ENCRYPTED,
+  R3_FLOW_CONTROL,
 
   R_IPO_IDENTIFIER,
   R_IPO_IP_ADDRESS,