Remove CancelAfter and Delay from internal TimeProvider

[martincostello]

Attempting to rebase #1144 for .NET 8 and to consume TimeProvider from NuGet (and use the new FakeTimeProvider implementation) things get complicated because there are various tests that rely on TimeProvider having a virtual CancelAfter() and Delay() method that can be overridden in the tests.

Polly/src/Polly.Core/ToBeRemoved/TimeProvider.cs

Lines 53 to 57 in a9bf3e9

	// This one is not on TimeProvider, temporarly we need to use it
	public virtual Task Delay(TimeSpan delay, CancellationToken cancellationToken = default) => Task.Delay(delay, cancellationToken);
	// This one is not on TimeProvider, temporarly we need to use it
	public virtual void CancelAfter(CancellationTokenSource source, TimeSpan delay) => source.CancelAfter(delay);

The real implementation instead has extension methods (which we can't mock) which are TimeProvider.Delay() and TimeProvider.CreateCancellationTokenSource().

To make the switch over for .NET 8 simpler, we should sync the current internal implementation to be source compatible with the .NET 8 rather than rely on methods which won't exist. That way, the tests will not need to change much and the switch over should be a lot cleaner.

[martincostello]

Bonus of trying to use use FakeTimeProvider in our tests is that I found this bug: dotnet/extensions#4114

[martintmk]

Thanks for reporting this. We should definitely sync the implementations to reduce the size of #1144 PR.

Our TimeProvider should include the CreateTimer method:
https://github.com/dotnet/runtime/blob/cfe30c057debbdd9ad21b36ae61062dbe72a1f83/src/libraries/Common/src/System/TimeProvider.cs#L156

We should also port the extensions:
https://github.com/dotnet/runtime/blob/cfe30c057debbdd9ad21b36ae61062dbe72a1f83/src/libraries/Microsoft.Bcl.TimeProvider/src/System/Threading/Tasks/TimeProviderTaskExtensions.cs#L13

All above made as internal and in a single file to simplify the sharing. Once .NET 8 is out we can nuke it and just use the standard package on lower frameworks.

.NET 8 target will still have no dependencies.

[martincostello]

Yeah I was trying to do that but I got stuck once I tried to refactor the tests to account for not using those members added extra 😅

If next week you could take a look at that then I can rebase #1144 onto main and slim it down.