Request image support

[sixlive]

Description

Closes #28

This PR adds image sending support for Prism and all first-party providers that support images. To accomplish this there was a pretty heavy refactor to the way messages work. It should be mostly backwards compatible.

Note: not all providers support all types of images, I tried to handle this by throwing an exception when unsupported methods are used.

Usage

Prism::text()
    ->using(Provider::OpenAI, 'gpt-4o')
    ->withMessages([
        new UserMessage(
            'What is this image',
            additionalContent: [
                Image::fromPath(storage_path('logo.png')),
                Image::fromBase64(base64_encode(storage_path('logo.png')), 'image/png'),
                Image::fromUrl('https://storage.echolabs.dev/assets/logo.png'),
            ],
        ),
    ])
    ->generate();