Add models endpoint

[yannickgloster]

Anthropic recently introduced their models endpoints.

This PR adds functions for:

• listing models
• fetching details on a specific model
• specs for the above
• README updates

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?

[crmne]

I find this quite important as companies release many models, update, and deprecate current ones (they just announced the deprecation of claude-2 models).

[yannickgloster]

@alexrudall any chance this could get reviewed and merged to main?