net: lwm2m: Improve token generation

Improve token handling by removing special meaning of tokenlen == 0,
which allows to handle server requests w/o a token (so far such
requests would cause the lwm2m engine to autogenerate token in the
response).

In order to autogenerate token during message initialization, use
special symbol `LWM2M_MSG_TOKEN_GENERATE_NEW`. If no token is wished to
be used, simply set the tokenlen to 0.

Additionally, fix an issue with token autogeneration, where invalid
token len was used (0 instead of 8).

Fixes #28299

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>