Define roles to let users use application CRDs dynamically

[ca-scribner]

We currently define the roles for different applications statically, where if we add an application we create new roles in this operator. This is irrespective of whether someone has actually deployed this application. We should change this to instead be dynamic and tailored to what has been deployed.

A concept for this could be through this charm providing a user-role relation (or something with a better name :) ). Applications could relate to this relation and provide ClusterRoles like these that define everything that should be included. This operator could then be in charge of maintaining those roles. This way if a Seldon is deployed and kfp is not, roles only exist for Seldon. And if a new Seldon release adds a new CRD, it is managed in what the Seldon charm provides to the user-role relation, rather than here.