Inject TimeProvider for testable, UTC-based expiration

[matthewdevenny]

Problem

The cache uses DateTimeOffset.Now throughout for expiration logic (src/NatsDistributedCache/NatsCache.cs — TTL computation, entry creation, absolute-expiration checks, sliding-window updates). This uses local time and makes expiration behavior impossible to unit-test without real wall-clock delays.

Proposal

Inject TimeProvider (default TimeProvider.System) into NatsCache and replace every DateTimeOffset.Now with _timeProvider.GetUtcNow().

This is the .NET 8+ idiom and enables deterministic, instant expiration unit tests via FakeTimeProvider (Microsoft.Extensions.TimeProvider.Testing), removing reliance on Task.Delay in the time-expiration tests.

Acceptance criteria

• No DateTimeOffset.Now remains in the library.
• Time-expiration unit tests drive time with FakeTimeProvider (no real delays).
• DI wiring resolves an optional TimeProvider, defaulting to TimeProvider.System.