Implement `starknet.rs` support for contract class endpoints

[jelilat]

Implement autogenerated starknet.rs support for the following endpoints.

• starknet_getClass
• starknet_getClassHashAt
• starknet_getClassAt

Additional Context:

• https://github.com/NethermindEth/rpc-request-builder/issues/2
• https://github.com/NethermindEth/rpc-request-builder/blob/main/src/api/rpcspec/methods.ts

[saimeunt]

I'd like to implement this one please.

[jelilat]

I'd like to implement this one please.

Assigned to you @saimeunt