Provide an emulator for local development and integration tests

### Is your feature request related to a problem? Please describe.
It's now not possible to develop or run integration tests locally without an internet connection.

### Describe the solution you'd like
An emulator (similar to what's available for SignalR) should make local development and integration tests easier and less cumbersome. Additionally it could also be included in the Aspire .NET Azure Web PubSub integration.

No files, tests, or entry points are named. Start by reviewing the repository structure and how the existing Web PubSub service is used in integration tests, then compare the requested behavior with the SignalR emulator and Aspire integration; done should include a defined local emulator that supports offline development and integration testing.

Written by the indexing model from the issue text.