[net/http auth] Use strings to identify authentication schemes.

[net/http auth] Use strings to identify authentication schemes.

While the network stack allowed registration of arbitrary authentication
schemes, the authentication logic relied on being able to use the
HttpAuth::Scheme enumeration to uniquely identify each scheme. In
practice, this restricted the list of supported authentication schemes
to those that were well known and supported by the network stack.

This change allows the use of any RFC 7235 compliant scheme for an
authentication handler. It also consolidates the determination of
relative priorities. Previously, even though HttpAuthHandlers could be
implemented externally to //net, they had to expose a priority score
that's in reality an implementation detail of supported authentication
schemes.

In addition, consolidation of authentication handler priorities alows
HttpAuth to score authentication challenge headers without constructing
HttpAuthHandlers for each header.

BUG=270219
BUG=529321