net: lwm2m: Add application/link-format content writer

This commit adds a new content writer, application/link-format, which
can be used during the Discovery procedure, to fill the content of the
response payload.

Introducing this new content writer, which encapsulates some of the
details like attribute handling which is different for bootstrap/regular
discovery, allows to unify the discovery handler in the lwm2m_engine,
thus it's no longer needed to have spearate handler functions for
bootstrap/regular discovery.

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