v1.0.0-preview.5

Aligned the public API surface of ManualTimeProvider with Microsoft.Extensions.Time.Testing.FakeTimeProvider. This means:

• The StartTime property is now called Start.
• The ForwardTime method has been removed (use Advance instead).
• The AutoAdvanceAmount property has been introduced, which will advance time with the specified amount every time GetUtcNow() is called. It defaults to TimeSpan.Zero, which disables auto-advancing.