net: lwm2m: Make query buffer large enough to encode all query strings

Make sure query string used by the lwm2m_rd_client is large enough to
encode any query string that can be sent during bootstrap/registration.
As the maximum query string length is related to the endpoint name,
which is limited by `CONFIG_LWM2M_RD_CLIENT_ENDPOINT_NAME_MAX_LENGTH`,
make the query string corellated to the value of this config.

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