Use managed identities for SQL server admin + schema creation

[audols]

We are currently using an AAD security group (that adds the user making the deployment) as an admin to the bicep-created SQL Server. Ideally we could conduct all auth through a user-assigned managed identity to ensure a higher degree of security.

Tasks

• Add bicep deployment for user-assigned managed identity
• Make MSI the SQL server admin (either as security group member or as direct admin)
• Update Excitation repo's data-source.ts to use a managed identity rather than the DefaultAzureCredential