feat(protocol): enforce initializer call with onlyInitializing modifier

[arularmstrong]

This PR strengthens code security by adding the onlyInitializing modifier to several init methods, ensuring they are called only from functions with an initializer modifier. Additionally, it improves code clarity by directly importing Initializable rather than relying on child inheritance. These changes enhance contract integrity and maintainability.

The init methods lacking the onlyInitializing modifier are as follows:

1. __AddressResolver_init()
2. __Essential_init()
3. __OwnerUUPSUpgradable_init()
4. __CrossChainOwned_init()
5. __MerkleClaimable_init()