Support multiple OAuth flows in ForceClient.cs via IAuthProvider

[DarkTigerFAF]

Hello, Anthony

Currently, ForceClient.cs can only log in with the username–password flow. This makes it difficult to use other flows (Web Server, Client Credentials, Refresh Token) that are already implemented in AuthenticationClient.cs.

Proposal: introduce a simple IAuthProvider strategy interface. Each provider wraps the existing AuthenticationClient methods and returns an AccessTokenResponse. ForceClient consumes the result and initializes itself.

Would you like me to open a PR to implement this?

[anthonyreilly]

Sounds like a good idea - if you have time to submit a PR that would be great, and I'll get it in