Create an interface for the main contract

[QGarchery]

A nice feature would be to have an interface for this liquidation protection contract. If the mapping subscriptions is public, then we should use the methodology of morpho-blue where we can fetch structs directly. So this would mean that instead of doing

we could simply do assertEq(params, liquidationProtection(subscriptions(subscriptionId)));
(this example is taken from #3)
I can do the PR on top of #3 if we want to go for it

Originally posted by @QGarchery in #1 (comment)

[QGarchery]

Completed in abce0a0