Zero-credential-storage architecture. The broker holds ACs
(authorization). Pluggable CredentialBackend implementations
hold secrets. Transports acquire short-lived, scoped credentials
at invocation time and discard after use.
Credential types: BasculeCredential, KerberosCredential,
OAuthCredential, SSHCertCredential.
Signed-off-by: Tyler King <tking@guildhouse.dev>
