Design and implement support for registered tags

[PJK]

http://www.iana.org/assignments/cbor-tags/cbor-tags.xhtml

[mofosyne]

Been giving the idea of an autogenerated C header list for cbor tags in https://github.com/mofosyne/iana-headers/

https://github.com/mofosyne/iana-headers/blob/main/cbor/c/cbor-constants.h

Happy to adapt this to libcbor if you have some idea how it can best fit in it.

While the code generator is written, at the moment the header style and naming convention is still in flux as it's not used in any major projects yet. So will be happy to adjust my project to match a sensible style.