Add ModelContextProtocol.Hosting package

[halter73]

We should introduce a ModelContextProtocol.Hosting package that is basically the same as what ModelContextProtocol was previously before b9f603a and depends on the ModelContextProtocol package as it is in this PR. So, we'd have three packages:

• ModelContextProtocol
  • Depends on Microsoft.Extensions.AI.Abstractions
  • Depends on Microsoft.Extensions.Logging.Abstractions (and thereby M.E.DI)
  • Depends on System.Net.ServerSentEvents
• ModelContextProtocol.Hosting
  • Depends on ModelContextProtocol
  • Depends on Microsoft.Extensions.Hosting.Abstractions (and thereby Options, Diagnostic.Abstractions, etc...)
• ModelContextProtocol.AspNetCore
  • Depends on ModelContextProtocol.Hosting
  • Depends on Microsoft.AspNetCore.App FrameworkReference