feat: Milestone 1 - CRD Foundation for Manager-Worker model

[josephaw1022]

This PR implements Milestone 1 of the Manager-Worker architecture proposal as described in issue #86.

Key changes:

• Introduced shared IdentitySpec for multiple authentication types (SqlLogin, WorkloadIdentity, AppRegistration).
• Updated SQLServer and ExternalSQLServer to include the Identity spec.
• Refactored existing controllers and finalizers to use Identity.SecretName.
• Introduced ExternalDatabase CRD for database-level identity isolation (required for Azure SQL).
• Refactored Helm templates to use mustMerge/deepCopy to avoid duplicate keys.
• Standardized Kubernetes labels in Helm chart using selectorLabels/commonLabels.
• Updated C# unit tests (TestDataBuilder) to match the new CRD structure.
• Switched to LF line endings in .editorconfig and standardized all files to fix CI formatting failures.

Ref: #86