This is an alist of subtypes for the authorization header parser. A subparser of this kind accepts the string containing the header and an integer position in the string. It should parse from that position onwards, and return the parsed contents as an alist of header parameters. Usually, these are actually pseudo-parameters; they don't necessarily have to appear in parameter syntax in the header. The unparser should be configured to expect the same parameters and combine them back into a string, though.
This parameter defaults to:
`((basic . ,basic-auth-subparser) (digest . ,digest-auth-subparser))