Use the new TimeProvider API from .NET 8

[martintmk]

Instead of writing our own variant of IClock that allows mocking time in tests we can now leverage the TimeProvider API that will be introduced in .NET 8.

The TimeProvider will be also available in .NET Framework by referencing the new nuget package.

Until the package is ready we can just copy the implementaiton into Polly, mark all classes as internal and once the official package is ready expose ResilienceStrategyBuilderOptions.TimeProvider property.

Wit this approach we will be using unified time abstraction and also allow easy unit-testing of strategies working with time.

[github-actions]

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