Cross Flow `MappingFn`s: Look up value from a different flow within the same profile

[azriel91]

Enables developers to define infrastructure in one flow, and have values from that flow automatically accessible to other flows, easing the propagation of values.

Something like:

let server_spec = Server::field_wise_spec()
    .with_vpc_from_flow(
        flow_id!("env_deploy"),
        item_id!("profile_vpc"),
        |vpc: &Vpc| vpc.map(Vpc::Id)
    )
    .await?;