Make OAuth token server configurable

[flyrain]

Java and Python clients support configurable OAuth token server. The Rust client needs that as well.

Here is an pyIceberg configuration example

catalog:
  default:
    uri: http://localhost:8082/
    rest.authorization-url: https://xxx.com/oauth2/token
    credential: client_id:client_secret

cc @RussellSpitzer @liurenjie1024 @himadripal

[whynick1]

@flyrain if nobody is already looking at this, would love to try and take a stab at this?

[flyrain]

Great! Assigned this to you. Thanks for taking a look.