protocol: add support for 'authtype' capability

[becm]

switch to alternative credential properties for 'authtype'
add support for 'ephemeral' state to protocol and basic credential interface

[becm]

This lays the required ground work for an improved multi-token provider data flow:

• move all token verify and storage operations to get phase to avoid races or storage target mismatch
• emit access_token credentials as ephemeral
• do not act on ephemeral credentials in the respective store or erase phase
  (use state[] entry to pass on selected provider, no need to identify matching credential).

This would move away from the classic Git credential validation data flow.
Even if Git deems the provided token invalid, the provider already supplied its best (validated) result.

[mjcheetham]

Looks like a reasonable addition! Just one comment about the IsTruthy reimplementation.

[becm]

@mjcheetham location of the IsEphemeral flag honestly came down to a coin toss.

I would have no reservations to putting it next to the other flags in GitResponse.
Would only need to be where it is, if a (future) storage layer wants to make use of it.