Add function to get names of all supported TrustClaims

[fitzthum]

It's a little awkward to work with the trust claim names. It would be handy if the crate had an interface to list all of the supported trust claims. This allows the consumer of the crate to iterate through the claims without having to keep their own list, which could get out of sync.

Also, it seems like it might be good to track these with an enum. You can use strum to convert that into strings and back.