Flatten contracts instantiation

[maurolacy]

#66 follow-up, and part of #45.

Let's simplify the instantiation logic further, by making the SDK instantiation logic explicit, instead of the babylon contract instantiating the other contracts.

After closer analysis this isn't as simple as it looks like, as there are cyclic dependencies between contracts

1. babylon-contract depends on both btc-staking and btc-finality.
2. btc-staking depends on babylon-contract.
3. btc-finality depends on both babylon-contract and btc-staking.

So, we would need either a custom query to provide these addresses to contracts, as proposed in babylonlabs-io/babylon-contract#94, or a (privileged) way to set these addresses in the contracts (a gated set_contracts_addresses() helper in babylon-contract). Or perhaps, to stick to the current instantiation logic. On second thought, it's clearly not so bad compared to all these requirements for handling deps across contracts.