Introduce TimeProvider in place of DateTimeHandler

[JanHyka]

Issue #, if available:

#286

Description of changes:

Initial step allowing implementation of #286.

This pull request updates the AWS.Messaging library to replace the custom DateTimeHandler abstraction with the .NET 8 TimeProvider for handling time-related functionality. This change modernizes the codebase, improves testability, and removes now-unnecessary code. Several classes and their constructors have been updated to use TimeProvider, and related tests and dependency registrations have also been adjusted.

Key changes include:

Migration to .NET 8 TimeProvider:

• Replaced all usages of IDateTimeHandler and DateTimeHandler with the built-in TimeProvider throughout the codebase, including in EnvelopeSerializer, SingleTypeSqsPollerEnvelopeSerializer, BackoffHandler, DefaultMessageManager, and InFlightMetadata.

Removal of Obsolete Code:

• Deleted the now-unnecessary DateTimeHandler and IDateTimeHandler files and interfaces.
• Constructors of public API classes missing TimeProvider internally use TimeProvider.System and are marked as [Obsolete] - this is to reduce risk of unexpected behavior in case custom TimeProvider is used and propagation chain gets silently broken.

Dependency Injection and Constructor Updates:

• Updated dependency injection registrations and factory methods to provide TimeProvider.System where required. Added new constructor overloads and marked old ones as obsolete to ease migration.

Test Adjustments and Improvements:

• Updated unit tests to use the new TimeProvider abstraction, including using FakeTimeProvider for improved testability. Added the Microsoft.Extensions.TimeProvider.Testing NuGet package to test projects.
• Work around thread safety issue in TestLambdaLogger, allowing significant unit test speedup
• Unit test accumulative duration drop to ~50% (1.2min -> 38.4s)

Changelog and Documentation:

• Added a changelog entry indicating the migration from DateTimeHandler to .NET 8 TimeProvider.

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