Implement minimal support for chained credential store helpers

[mschlumpp]

Tools such as git-credential-oauth rely on a chained helper configuration to store the acquired credentials.

Unimplemented features in this PR:

• This does not pass on properties other than username and password (Notable other properties would be password_expiry_utc, oauth_refresh_token).
• It does not check the password_expiry_utc property, and instead relies on the timeout config of the cache helper.

Note: git-credential-oauth does not seem to make use of these features at the moment.