Microsoft.Azure.Functions.Worker.Extensions.SignalRService 1.15.0

Important: due to a common issue #2776 in dotnet isolated worker, this version can't use managed identity to connect to Azure SignalR Service until #2776 is fixed.

What's Changed

• Support SignalR trigger return value by @Y-Sindo in #2771

Full Changelog: signalr-extension-1.14.1...signalr-extension-1.15.0