Reconsider IA2AClient, ITaskManager, ITaskStore interfaces

### Context:
The A2A SDK declares a few public interfaces: `IA2AClient`, `ITaskManager`, `ITaskStore`, and corresponding classes that implement them. While having the interfaces may be quite convenient for polymorphic (when different implementations are required) and testability (mocking a class for testing purposes) cases, exposing the interfaces as part of the public API surface of SDKs may have downsides, such as a source of breaking changes for all third-party implementations that SDK authors are not aware of when new members are added to them.

### ToDo:
1. Identify all public interfaces in the SDK.
2. For each interface, consider:
- Whether it is actually required, and if not, consider removing it.
- Whether it can be turned into an abstract class. New members can easily be added to abstract classes as virtual without breaking existing implementations, but there could be a downside in case a consumer's class already inherits from another class - C# does not allow multiple inheritance.