Upgrade Azure.Identity to 1.15.0+ to support environment variable credential filtering

[Maciej-Rakocki]

Description:

Problem

The backend's Azure.Identity dependency (1.11.4) does not support environment variables for credential exclusion (AZURE_TOKEN_CREDENTIALS, exclude_managed_identity_credential). This forces developers in restricted networking environments to either use firewall rules to block Managed identity credentials or rebuild the backend.
https://learn.microsoft.com/en-gb/dotnet/azure/sdk/authentication/credential-chains?tabs=dac#use-a-specific-credential

Solution

Add an explicit Azure.Identity 1.15.0 or newer package reference in durablefunctionsmonitor.dotnetbackend.csproj:

Benefits

• Developers can set $env:AZURE_TOKEN_CREDENTIALS='dev' to exclude Managed Identity in local dev.
• Developers can select credential method explicitly through environment variables