Adopt a plugin architecture for Flows #140
Description
Currently grant types are executed by dedicated Flow instances, e.g. a client_credentials grant is executed by the ClientCredentialsFlow.
The set of supported grants is thus statically defined by the number of implemented Flow instances.
By opening this to a pluggable architecture (SPI / Service Loader for example), we could keep supporting the essential grants out of the box while allowing users to add support for custom grants, or for behavioral variants of existing grants.