feat: add recommendations for variable validation and optional objects

[RoseSecurity]

Why

• This introduces examples of optional object attributes and comprehensive variable validation techniques. These updates help new users define more flexible variables with default values and ensure input correctness through validation blocks.

What

• Updated documentation in writing-terraform-configurations.md to include:
  • Examples of optional object attributes in variables, showing how to provide defaults for non-required fields.
  • A new section on variable validation, including:
    • Basic validation for string variables (e.g., restricting environment names).
    • Validation for objects (e.g., restricting allowed database engines).
    • Validation for lists (e.g., ensuring all CIDR blocks are valid).