AggLayer: Encapsulate the creation of agglayer faucet and bridge in structs with `new` constructor

[mmagician]

this is nice and contained. I'd love to see a similar pattern done for create_agglayer_faucet_component, where we have a struct and a way to convert it into an AccountComponent easily. And then we could avoid hardcoding the metadata layout and use sth like to_metadata_word here.

pub struct AggLayerFaucet {
   // ...
}

impl From<AggLayerFaucet> for AccountComponent {
    // ...
}

I'll open an issue to track this.

(and same for AggLayerBridge)

Originally posted by @mmagician in #2335 (comment)