Create the class AlastriaProvider

[VictorNS69]

The idea is to create a class AlastriaProvider which sets the provider the user want (web3, ehter, ...) and checks if this provider has the functions we use in the lib (functions as .eth.abi.encodeFunctionCall), so if any of those functions does not exist, this class will throw an error saying that "this provider is not suported" (yet?).

Also, this class will create a default provider if none is passed as argument. This means to create a new account too.

Then, the lib functions need to be modified to receive an AlastriaProvider instance. Also, this new argument should be the last argument in the function as it will be optional (if none setted, default is created as explained before).

[VictorNS69]

Issue currently blocked because there are other issues with higher priority.

[VictorNS69]

If we migrate to ether-js, this issue is not needed.

See #120