Security improvement by moving away from "DefaultAzureCredential"

[doormalena]

The following is written on https://learn.microsoft.com/en-gb/dotnet/api/azure.identity.defaultazurecredential?view=azure-dotnet stating that "DefaultAzureCredential" is not to be used in production scenarios.

Simplifies authentication while developing apps that deploy to Azure by combining credentials used in Azure hosting environments with credentials used in local development. In production, it's better to use something else. See [Usage guidance for DefaultAzureCredential](https://aka.ms/azsdk/net/identity/credential-chains#usage-guidance-for-defaultazurecredential).

Consider allowing the user of AzureSignTool to specify which credential is to be used via a new additional parameter related to --azure-key-vault-managed-identity. For example, my needs would be the "WorkloadIdentityCredential". For others, this could be InteractiveBrowserCredential or anything else.