Use TimeProvider instead of DateTime ambient context

[alexmurari]

Currently, token lifetime validation is performed by getting the current time from a DateTime ambient context, as seen here.

(I know there's other places where DateTime is used to get the current time, this is just an example)

This leads to flaky, non-deterministic tests, as there's no control of the current time whatsoever.

With the introduction of TimeProvider in .NET 8, there's an out-of-the-box solution for abstracting time.

My proposal here is adopt TimeProvider instead of relying directly on static DateTime class.