Wish Item: Auth Aggrogate for Password Grant

[weierophinney]

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Would be nice to be able to provide our own auth method for the password grant, while I could certainly pass in a new userRepository, it would be nice to be able to add an additional password verifier to the existing repo. Perhaps we could make this an aggrogate service, much like zend-auth uses?

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Originally posted by @wshafer at https://github.com/zendframework/zend-expressive-authentication-oauth2/issues/25

[weierophinney]

This request can fall into this request: Create event hooks for all OAuth2 Repository functions.
Though my work with api-skeletons/zf-oauth2-doctrine I often see cases for custom authorization such as to validate against an old password hash or to validate against a PIN instead of a Password.

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Originally posted by @TomHAnderson at https://github.com/zendframework/zend-expressive-authentication-oauth2/issues/25#issuecomment-373133733