Add declare function to the generated Contract type

[glihm]

To ease the declaration of a class, the generated Contract type may expose a declare function.

abigen!(MyContract, "/path/sierra.json");

fn main() {
    // init provider and account

    let class_hash = MyContract::declare(...args).await?;
}

There are two possible strategies:

1. Taking only the Sierra class as input, but this enforce to re-compile the CASM class to compute the class hash (simpler interface).
2. We can accept the CASM class as optional parameter, to have the hash out of the box (faster).

Example for the declare can be taken here on Starkli.

The fees must be configurable.
We also may want to check if the class is not already declared, to accelerate the process in the case it is.

In the code, this is located in the rs crate here where a new file may be added for this, to be expanded in the contract type here.