core: expose the version info capability of IPv4 forwarding support

This commit adds NM_VERSION_INFO_CAPABILITY_IPV4_FORWARDING to the
VersionInfo D-Bus property, allowing clients such as nmstate to check
the NetworkManager's support of configuring per-device IPv4 sysctl
forwarding setting directly via the capabilities bitmask instead of
relying on the NetworkManager version comparisons.

src/core/nm-manager.c

@@ -478,7 +478,8 @@ _version_info_get(void)
          * Each of the array's elements has 32 bits. This means that capabilities
          * with index 0-31 goes to element #1, with index 32-63 to element #2,
          * with index 64-95 to element #3 and so on. */
-        1 << NM_VERSION_INFO_CAPABILITY_SYNC_ROUTE_WITH_TABLE,
+        (1 << NM_VERSION_INFO_CAPABILITY_SYNC_ROUTE_WITH_TABLE)
+            | (1 << NM_VERSION_INFO_CAPABILITY_IP4_FORWARDING),
     };
 
     return nm_g_variant_new_au(arr, G_N_ELEMENTS(arr));

src/libnm-core-public/nm-dbus-interface.h

@@ -100,6 +100,9 @@
  *   https://issues.redhat.com/browse/RHEL-66262
  *   https://issues.redhat.com/browse/RHEL-67324
  *
+ * @NM_VERSION_INFO_CAPABILITY_IP4_FORWARDING: Indicates that NetworkManager supports
+ * configuring per-device IPv4 sysctl forwarding setting. Since: 1.56.
+ *
  * The numeric values represent the bit index of the capability. These capabilities
  * can be queried in the "VersionInfo" D-Bus property.
  *
@@ -107,6 +110,7 @@
  */
 typedef enum {
     NM_VERSION_INFO_CAPABILITY_SYNC_ROUTE_WITH_TABLE = 0,
+    NM_VERSION_INFO_CAPABILITY_IP4_FORWARDING        = 1,
 } NMVersionInfoCapability;
 
 /**