add support for managed identity to connect to SQL Server

[gbissio]

Currently, Microsoft Fabric supports authentication via Service Principal, but Managed Identities are not yet supported.
Adding support for Managed Identities would bring significant benefits:

Eliminate the need to store or rotate secrets (passwords/keys) in connection strings.

Align Fabric with SQL Server 2022, which already supports Managed Identities.

Improve security and simplify governance by relying on Azure AD native identity management.

This would make Fabric more consistent with the broader Microsoft ecosystem and reduce operational overhead for customers.