feat(contracts): add file output type #329

Add the shared file contract output type to ContractOutputType in pyoaev.

Injectors need a file output type to emit file findings (e.g. NetExec spider_plus enumerating files on SMB shares, or --ls on FTP/NFS). The type must exist in this shared package before any injector can declare it, and before the platform can deserialize the contract.

Step 2 of the end-to-end file finding work for the attack-path feature (platform enum → client-python → output processor / chaining → injector).

Tracks the client-python side of OpenAEV-Platform/openaev#6647.

Implemented by #322.