network: use request queue to configure addresses, routes, and nexthops

Why is this necessary? Several examples below.

- When a route sets prefsrc, then the address must be already assigned
  (see issue #19285), and also it must be ready if IPv6.
- When a route or nexthop sets gateway, then the address must be reachable.
- When a route sets nexthop ID, then the corresponding nexthop must be
  assigned.
- When a route sets multipath routes on another interface, then the
  interface must exist and be ready to configure.
- When configuring address, the same address must not be under removing
  (see issue #18108).
Etc,. etc,...

So, this makes all requests about addresses, routes, and nethops are once
stored in the queue, and will be processed when they are ready to configure.

Fixes #18108 and #19285.

src/network/networkd-queue.h

@@ -5,7 +5,10 @@
 
 #include "networkd-link.h"
 
+typedef struct Address Address;
 typedef struct Neighbor Neighbor;
+typedef struct NextHop NextHop;
+typedef struct Route Route;
 typedef struct RoutingPolicyRule RoutingPolicyRule;
 
 typedef struct Request Request;
@@ -14,7 +17,10 @@ typedef int (*request_after_configure_handler_t)(Request*, void*);
 typedef void (*request_on_free_handler_t)(Request*);
 
 typedef enum RequestType {
+        REQUEST_TYPE_ADDRESS,
         REQUEST_TYPE_NEIGHBOR,
+        REQUEST_TYPE_NEXTHOP,
+        REQUEST_TYPE_ROUTE,
         REQUEST_TYPE_ROUTING_POLICY_RULE,
         _REQUEST_TYPE_MAX,
         _REQUEST_TYPE_INVALID = -EINVAL,
@@ -25,7 +31,10 @@ typedef struct Request {
         RequestType type;
         bool consume_object;
         union {
+                Address *address;
                 Neighbor *neighbor;
+                NextHop *nexthop;
+                Route *route;
                 RoutingPolicyRule *rule;
                 void *object;
         };