core: limit number of LogExtraFields (#41044)

We have two places where those fields can be set: config and the dbus
interface. Let's clamp down on the number in both places. But in
principle, we could also be upgrading (through
serialization/deserialization) from an older systemd which didn't
enforce this limit, so also check on deserialization. A user could have
a unit with lots and lots of ExtraFields, but not enough to cause the
issue in #40916. To handle this gracefully, ignore the extra fields,
like we do in the parser. Where the field is used, assert that we are
within the expected bounds.

Fixes #40916.

Reproducer:
```console
$ python3 -c 'from pydbus import SystemBus; from gi.repository import GLib; SystemBus().get("org.freedesktop.systemd1", "/org/freedesktop/systemd1").StartTransientUnit("crash.service", "fail", [("ExecStart", GLib.Variant("a(sasb)", [("/bin/true", ["/bin/true"], False)])), ("LogExtraFields", GLib.Variant("aay", [b"F%d=x" % i for i in range(140000)]))], [])' Traceback (most recent call last):
  File "<string>", line 1, in <module>
    from pydbus import SystemBus; from gi.repository import GLib; SystemBus().get("org.freedesktop.systemd1", "/org/freedesktop/systemd1").StartTransientUnit("crash.service", "fail", [("ExecStart", GLib.Variant("a(sasb)", [("/bin/true", ["/bin/true"], False)])), ("LogExtraFields", GLib.Variant("aay", [b"F%d=x" % i for i in range(140000)]))], [])
                                                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/site-packages/pydbus/proxy_method.py", line 102, in __call__
    raise error
  File "/usr/lib/python3.14/site-packages/pydbus/proxy_method.py", line 97, in __call__
    result = instance._bus.con.call_sync(*call_args)
gi.repository.GLib.GError: g-dbus-error-quark: GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs: Too many extra log fields. (16)
```