[xcm-emulator] Make a generic genesis constructor method

[0xmovses]

Currently we write a separate genesis function for each Parachain when creating the emulated test environments cumulus/parachains/integration-tests/emulated/chains/parachains/bridges/bridge-hub-rococo/src/genesis.rs.
Much of this logic is repeated, but some is chain / environment specific. We can make a generic function, reducing code repetition and making this code more expressive.

We could do something like

trait RuntimeGenesisConfig {
    fn configure_storage(storage: &mut Storage);
}

Then we could do a impl RuntimeConfigurator for BridgeHubRococoRuntime, for example.

And finally:

impl Storage {
    pub fn new_with_config<R: GenesisRuntimeConfig>() -> Self {
        let mut storage = Storage::default();
        R::configure_storage(&mut storage);
        storage
    }
}

Originally suggested here by @liamaharon

[0xmovses]

Opened in wrong repo