Move endpoints to Calendar-Assistant API project (#25)

### Summary & Motivation
Move the endpoints from the Core project to the Calendar-Assistant API
project because `IEndpoints` are only picked up by Dependency Injection
when they reside in the API project. This adjustment ensures that the
Dependency Injection system properly registers and manages these
endpoints.

### Atomic Changes
- Move endpoints to Calendar-Assistant API project

### Checklist

- [x] I have added a Label to the pull-request
- [x] I have added tests, and done manual regression tests
- [x] I have updated the documentation, if necessary

---------

Co-authored-by: Marco van Kimmenade <[email protected]>

application/calendar-assistant/Core/Conversations/ConversationsEndpoints.cs → application/calendar-assistant/Api/Endpoints/ConversationsEndpoints.cs

@@ -1,11 +1,8 @@
-using Microsoft.AspNetCore.Builder;
-using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.Routing;
 using Microsoft.Bot.Builder;
 using Microsoft.Bot.Builder.Integration.AspNet.Core;
 using PlatformPlatform.SharedKernel.Endpoints;
 
-namespace Moment42.CalendarAssistant.Conversations;
+namespace Moment42.CalendarAssistant.Api.Endpoints;
 
 public sealed class ConversationsEndpoints : IEndpoints
 {