OAuth client should support bi-phasic oauth flows

[vitorbal]

Description

The OAuthClientProvider interfaces should look more like the Typescript SDK's provider instead, which is less opinionated about how the code verifier and redirect uri are consumed and subsequently exchanged into a token.

Currently, the Python SDK's version makes assumptions about the end-to-end flow that prevent usage in scenarios where e.g. I want to store the verifier/uri in the DB and run the callback "out-of-band".

References

• https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/client/auth.ts
• https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/examples/client/simpleOAuthClient.ts