Support async/reactive style auth mecanism

OpenID shows that current API is too rigid and, the typical example is the token fetch which is typically a NIO operation but is done synchronously in current implementation so misuse the server resources.
Goal of this task is to ensure the API becomes more friendly to such needs.
Minimum is to expect the SPI return types to be CompletionStage IMHO.

Side note: I don't think depending on a new dependency to be reactive with a lot of verbose API would be good for the project, sticking to java is sufficient IMHO.

Start by reviewing the current authentication SPI and the OpenID token-fetch path described in the issue, focusing on where the synchronous operation is exposed. Determine the API changes needed for SPI methods to return CompletionStage without adding a reactive dependency. Done means the authentication API supports asynchronous token fetching while preserving the project’s Java-only approach.

Written by the indexing model from the issue text.