V2 - Expose token handling functions in the public API.

Some developers want to use the token handling parts of this library with their own hand-coded API calls. Some prefer a DIY approach or are porting an existing code-base with poor token handling.

Some developers want to use this library to make most API calls and then do one or two independently. They may need to optimize a hotspot, work around a limitation of this library, or use an experimental eBay feature. They want to use the same tokens for their calls with an automatic refresh.

Some developers want to do their own eBay authentication. Perhaps they have higher security requirements or their user management code with extra functions.

While accommodating the few, don't burden the many with extra work. Continue to make "convention over configuration" and "batteries included" the norm for most users.

Let us do potentially breaking changes before moving to production/stable.

Start by reviewing the library's existing token-handling and authentication entry points, then compare them with the public API surface. Define which token operations should be exposed for custom eBay calls, automatic refresh, and user-managed authentication; the work is done when those boundaries and compatibility expectations are specified and covered by tests.

Written by the indexing model from the issue text.