feat: allow optional injection of custom Pinecone client

[clioo]

• Add a new client parameter to PineconeIndex.__init__ for dependency injection.
• Use the supplied client if provided, otherwise initialize a new Pinecone client.
• Maintain backward compatibility while improving testability and configuration flexibility.
• removing duplication on api_key

Fixes #525

[jamescalam]

thanks for the pr @clioo — I see issues in the lint, are you able to take a look? The linter can be run locally with poetry run make lint

The test failure potentially looks like it is due to api keys — I'll test locally once linting is resolved