Add support for organization API

[sharifhsn]

Fixes #260

This PR currently only implements the invites module of the organization API. I'd like to get clarification on some details before I continue.

• The organization API requires a different API key, denoted as OPENAI_ADMIN_KEY in documentation. I'm thinking that I should write an additional method on OpenAIConfig to use this environment variable, perhaps with_admin_key. What do you think?
• All of the organization APIs are organized under that endpoint. As with the assistants module, I'm choosing to keep them in the flat namespace.
• I'm choosing to name the types based on their names in the OpenAPI schema. Is this correct, or should I follow a different schema?
• What is the minimum supported Rust version of this project? After checking with cargo-msrv, the current MSRV is 1.70.0. If this is intended, it should be indicated somewhere in the README.