netlink: specs: define ethtool header flags

When interfacing with the ethtool commands it's handy to
be able to use the names of the flags. Example:

    ethnl.pause_get({"header": {"dev-index": cfg.ifindex,
                                "flags": {'stats'}}})

Note that not all commands accept all the flags,
but the meaning of the bits does not change command
to command.
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240403023426.1762996-2-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/ethtool.yaml

@@ -16,6 +16,11 @@ definitions:
     name: stringset
     type: enum
     entries: []
+  -
+    name: header-flags
+    enum-name:
+    type: flags
+    entries: [ compact-bitsets, omit-reply, stats ]
 
 attribute-sets:
   -
@@ -30,6 +35,7 @@ attribute-sets:
       -
         name: flags
         type: u32
+        enum: header-flags
 
   -
     name: bitset-bit