Allow using custom account class hashes

[amanusk]

Descrbie the Problem

When creating a new account, starkli lets you select one of Argent, Braavos or OZ accounts. It uses the latest hash available from these accounts, and what was updated in the known accounts list Sometimes the lastest account class is not yet declared on every network, or there is a need to use a custom class hash, with similar ABI but changed functionality. It would be useful for starkli to allow to specify a custom account class hash upon creation.

Proposed interface

starkli account oz init --custom_class_hash <0x...>

This will assume the user knows what they are doing by specifying a specific account class hash when creating their account. (Potentially, it is also possible to check if the class has the same ABI, but that is extra)