Abstraction library for interfaces and base types

### Describe the feature

An AWS.Messaging abstractions library that has the interfaces and base types like IMessageHandler, MessageEnvelope, IMessagePublisher, etc. Something similar to the [MassTransit.Abstractions](https://www.nuget.org/packages/MassTransit.Abstractions/)

### Use Case

I'm often working in projects where external services are configured in a different layer than where the message handlers are implemented. It would be really nice to have a separate package where I can just get the base types from.

### Proposed Solution

I don't think it would have to be a breaking change if it was added as a dependency in the AWS.Messaging library. Of course the namespace would have to remain the same in the abstractions library.

### Other Information

_No response_

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS.Messaging (or related) package versions

AWS.Messaging 1.0.0

### Targeted .NET Platform

.NET 8

### Operating System and version

macOS Sequoia 15.5

Start by reviewing the existing AWS.Messaging 1.0.0 public abstractions and the referenced MassTransit.Abstractions package. Define the separate package boundary for IMessageHandler, MessageEnvelope, IMessagePublisher, and related base types while preserving their namespaces; done means these types are available from the new .NET 8 package without a breaking change.

Written by the indexing model from the issue text.