credential: add WWW-Authenticate header to cred requests

Add the value of the WWW-Authenticate response header to credential
requests. Credential helpers that understand and support HTTP
authentication and authorization can use this standard header (RFC 2616
Section 14.47 [1]) to generate valid credentials.

WWW-Authenticate headers can contain information pertaining to the
authority, authentication mechanism, or extra parameters/scopes that are
required.

The current I/O format for credential helpers only allows for unique
names for properties/attributes, so in order to transmit multiple header
values (with a specific order) we introduce a new convention whereby a
C-style array syntax is used in the property name to denote multiple
ordered values for the same property.

In this case we send multiple `wwwauth[]` properties where the order
that the repeated attributes appear in the conversation reflects the
order that the WWW-Authenticate headers appeared in the HTTP response.

Add a set of tests to exercise the HTTP authentication header parsing
and the interop with credential helpers. Credential helpers will receive
WWW-Authenticate information in credential requests.

[1] https://datatracker.ietf.org/doc/html/rfc2616#section-14.47

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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