Provide content type number as field in CoapContentFormat::Other

[pulsastrix]

          Hmm, not really related to this PR, but could you also have `Other` accept the respective Content-Format value here?

Originally posted by @JKRhb in #15 (comment)

[pulsastrix]

Apparently, one possible way of implementing this could also be replacing num_derive with num_enum, which seems to be the more widely used crate for this purpose anyways.

num_enum allows creating a variant that captures all other cases using the #[num_enum(catch_all)] attribute.