Expose time provider for TimeWindowFilter

[zhiyuanliang-ms]

Why this PR?

#558

Visible Changes

Now TimeWindowFilter.SystemClock is public.

DI usage:

builder.Services.AddSingleton<TimeProvider, CustomTimeProvider>();
builder.Services.AddFeatureManagement(); // feature manager builder will get TimeProvider service from DI container

If there is no registered TimeProvider service, SystemTimeProvider will be used.

Direct usage:

var mockedTimeWindowFilter = new TimeWindowFilter()
{
    SystemClock = mockedTimeProvider
};