Add default implementation for CredentialsAuthenticatable
This patch was authored and released by @0xTim.
Improves the experience for users writing web applications. Adds a ModelCredentialsAuthenticator to automatically conform Model types to CredentialsAuthenticatable and provide a middleware to use.
This can be used when logging in users via a web form, as shown in the tests. This also backfills some tests for ModelSessionAuthenticatable.
Also fixes a bug where the SessionAuthenticator was not using the provided DatabaseID
Docs here vapor/docs#576