Declare a LINKAUTH subprotocol version for #15055 (ed link handshake)

Closes ticket 20552.

changes/feature20552

+  o Minor features (ed25519 link handshake):
+    - Advertise support for the ed25519 link handshake using the
+      subprotocol-versions mechanism, so that clients can tell which
+      relays can identity themselves by Ed25519 ID.

src/or/or.h

@@ -2247,6 +2247,10 @@ typedef struct routerstatus_t {
    * accept EXTEND2 cells */
   unsigned int supports_extend2_cells:1;
 
+  /** True iff this router has a protocol list that allows it to negotiate
+   * ed25519 identity keys on a link handshake. */
+  unsigned int supports_ed25519_link_handshake:1;
+
   unsigned int has_bandwidth:1; /**< The vote/consensus had bw info */
   unsigned int has_exitsummary:1; /**< The vote/consensus had exit summaries */
   unsigned int bw_is_unmeasured:1; /**< This is a consensus entry, with

src/or/protover.c

@@ -292,7 +292,7 @@ protover_get_supported_protocols(void)
     "HSIntro=3 "
     "HSRend=1-2 "
     "Link=1-4 "
-    "LinkAuth=1 "
+    "LinkAuth=1,3 "
     "Microdesc=1-2 "
     "Relay=1-2";
 }

src/or/routerparse.c

@@ -2962,6 +2962,8 @@ routerstatus_parse_entry_from_string(memarea_t *area,
     rs->protocols_known = 1;
     rs->supports_extend2_cells =
       protocol_list_supports_protocol(tok->args[0], PRT_RELAY, 2);
+    rs->supports_ed25519_link_handshake =
+      protocol_list_supports_protocol(tok->args[0], PRT_LINKAUTH, 3);
   }
   if ((tok = find_opt_by_keyword(tokens, K_V))) {
     tor_assert(tok->n_args == 1);