credential: add an authtype field

credential: add an authtype field

When Git makes an HTTP request, it can negotiate the type of
authentication to use with the server provided the authentication scheme
is one of a few well-known types (Basic, Digest, NTLM, or Negotiate).
However, some servers wish to use other types of authentication, such as
the Bearer type from OAuth2.  Since libcurl doesn't natively support
this type, it isn't possible to use it, and the user is forced to
specify the Authorization header using the http.extraheader setting.

However, storing a plaintext token in the repository configuration is
not very secure, especially if a repository can be shared by multiple
parties.  We already have support for many types of secure credential
storage by using credential helpers, so let's teach credential helpers
how to produce credentials for an arbitrary scheme.

If the credential helper specifies an authtype field, then it specifies
an authentication scheme (e.g., Bearer) and the password field specifies
the raw authentication token, with any encoding already specified.  We
reuse the password field for this because some credential helpers store
the metadata without encryption even though the password is encrypted,
and we'd like to avoid insecure storage if an older version of the
credential helper gets ahold of the data.

The username is not used in this case, but it is still preserved for the
purpose of finding the right credential if the user has multiple
accounts.

If the authtype field is not specified, then the password behaves as
normal and it is passed along with the username to libcurl.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>