Feature/proxy requests

[simonmitchell]

This PR adds a more generic approach to proxying requests to the OpenAI API through channels other than direct URLSession from the app.

For example, if you used a particular networking library in the app you could conform your networking client to OpenAIRequestHandler and make the requests yourselves. Or if you were using firebase callable functions as your proxy, or any other kind of "serverless" approach that has its own SDK you could use that instead.

I have made a concerted effort to make this entirely backwards compatible, this has been done by typealiasing Config = URLSessionRequestHandler and adding a secondary initialiser to the OpenAISwift class which takes an instance of the protocol OpenAIRequestHandler so that users who are either using:

OpenAISwift(config: .makeDefaultOpenAI(apiKey: "my_key"))

or

OpenAISwift(config: OpenAISwift.Config.makeDefaultOpenAI(apiKey: "my_key"))

do not have any breaking changes, but users who want to implement their own request handling can use:

OpenAISwift(requestHandler: someRequestHandlerInstance)

I have tested this with our own integration of the OpenAISwift library and all existing functionality works the same. Note: this also includes the fix for 400 error due to adding Identifiable protocol to ChatMessage as seen here: #122

[simonmitchell]

I'm more than happy to add docs covering this to the README for this PR if required? 😄