Merge branch 'bc/credential-scheme-enhancement'

Merge branch 'bc/credential-scheme-enhancement'

The credential helper protocol, together with the HTTP layer, have
been enhanced to support authentication schemes different from
username & password pair, like Bearer and NTLM.

* bc/credential-scheme-enhancement:
  credential: add method for querying capabilities
  credential-cache: implement authtype capability
  t: add credential tests for authtype
  credential: add support for multistage credential rounds
  t5563: refactor for multi-stage authentication
  docs: set a limit on credential line length
  credential: enable state capability
  credential: add an argument to keep state
  http: add support for authtype and credential
  docs: indicate new credential protocol fields
  credential: add a field called "ephemeral"
  credential: gate new fields on capability
  credential: add a field for pre-encoded credentials
  http: use new headers for each object request
  remote-curl: reset headers on new request
  credential: add an authtype field