Adopt TimeProvider abstraction when its ready

[stephentoub]

The DefaultHttpRetryHandler type defines IDelayProvider and ITimeProvider interfaces. The APIs these surface are fully covered by the new System.TimeProvider abstract type being introduced in .NET 8 but that will also be available from the Microsoft.Bcl.TimeProvider nuget package downlevel. Once it's available, it should ideally be used instead of Semantic Kernel defining its own abstractions.

[dluc]

Agreed, once we have a chance to revisit the unit tests approach, we'll start using this, removing the bespoke version