Add additional HTTP authorization types

[mgritter]

This includes:

• all registered types from IANA/IETF
• a mechanism for marking proprietary headers as authorization
• non-IANA types such as AWS and Microsoft (NTLM) that appear in the Authorization header

[liujed]

I think consumers of the IR currently assume that these authorization types are associated with the Authorization header. We'll need to weed out all the places that make this assumption.

[thatplguy]

The conversion to/from OpenAPI2/3 will need to be updated, as well as the translation of the IR -> frontend data structures. Maybe the visitors as well, @liujed? Seems doable though.

[liujed]

As I recall, parts of the GitHub comment generator will need to be updated too. Certainly doable; just need to be careful to examine all the places where we look at these values.

[thatplguy]

The conversion to/from OpenAPI2/3 will need to be updated, as well as the translation of the IR -> frontend data structures. Maybe the visitors as well, @liujed? Seems doable though.