feat: add the `WithTokenSource` client option

Add the `WithTokenSource()` client option to support custom
`oauth2.TokenSource` implementations for automatic token refresh and expiration
handling. The `TokenSource` is called on every API request, allowing for
flexible token management strategies. To reduce latency and network load, wrap
token sources in `oauth2.ReuseTokenSource`.

This enables proper OAuth 2.0 token lifecycle management and allows users to
use other OAuth flows, such as the Device Authorization Grant flow.