Add support of "ct=" attribute at Server side

[sbernard31]

Add support to ct= attribute at Server side.
"ct" attribute allow to let know to the server content formats supported by the client.

If the LwM2M Client supports optional data formats, it MAY inform the LwM2M Server by including the content type in the root path link using the ct= link attribute. (note that the content type value 110 is the value assigned in the CoAP Content-Format Registry for the SenML JSON format used by LwM2M).

</>;ct=110, </1/0>,</1/1>,</2/0>,</2/1>,</2/2>,</2/3>,</2/4>,</3/0>,</4/0>,</5>

(source core§6.2.1)

Since LWM2M 1.1, there is no more one mandatory content format to rule them all (TLV) and so this kind of negotiation is more needed.
(see OpenMobileAlliance/OMA_LwM2M_for_Developers#514)

[msangoi]

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