Olexandr Berezan

This PR resolves [#214](https://github.com/starkware-libs/starknet-specs/issues/214)

Since `flat` kind was added to [Cairo compiler](https://github.com/starkware-libs/cairo/blob/main/crates/cairo-lang-starknet-classes/src/abi.rs#L170) it should be also added to [ABI specs](https://github.com/starkware-libs/starknet-specs/blob/master/api/starknet_metadata.json#L422).