creds: hold credentials

creds: hold credentials

Authorizdation credentials are kept in `struct Curl_creds`. This contains:

* `user`: the username, maybe the empty string
* `passwd`: the password, maybe the empty string
* `sasl_authzid`: the SASL authz value, maybe the empty string
* `oauth_bearer`: the OAUTH bearer token, maybe the empty string
* `source`: where the credentials from from
* `refcount`: a reference counter to link/unkink creds

A `creds` with all values empty is equivalent to NULL, e.g. no `creds`
instance. With reference counting, `creds` can be linked/unlinked
in several places.

See docs/internals/CREDENTIALS.md for use.

Closes #21548