eventually-core: add Projection trait

[ar3s3ru]

Add support for projections by creating a Projection trait to define how to calculate the projection state based on the interested domain events and previous state:

pub trait Projection: Default { // There must be a default state
    type Event;
    type Error;

    // Projections are always applied to persisted events coming from the Subscription.
    fn project(self, event: store::Persisted<Self::Event>) -> Result<Self, Self::Error>;
}