Add support for variables in Control Sets to allow dynamic customization when deploying them across different spokes or clients.

The idea is to define variables in the parent Control Set (for example {client_name}) that can automatically be replaced with the appropriate value when the Control Set is deployed to a spoke.

Example use case:
A Control Set created at the parent level could contain standardized controls referencing {client_name}. When deployed to a spoke, the system would automatically replace {client_name} with the actual company name of that spoke.

Benefits:

Enables reusable and scalable Control Sets across multiple clients.

Reduces manual editing after deployment.

Improves consistency in control documentation.

Facilitates multi-tenant governance and compliance management.

Example:

Parent Control:
The organization {client_name} must maintain an asset inventory.

Deployed in spoke "ACME Inc":
The organization ACME Inc must maintain an asset inventory.