feat: add VectorStoreFiles API to Facade

[alexmayo]

Add VectorStoreFiles API to facade.

[iBotPeaches]

Am I misunderstanding how this works? I don't see vectorStoreFiles in the client which I thought is what this facade points to - https://github.com/openai-php/client/blob/main/src/Contracts/ClientContract.php

[iBotPeaches]

Yeah on investigation. I don't think this is right. You get files from

vectorStores()->files()

or

vectorStores()->batches()

for batches.