fix: Updated the object-based configuration in variable.tf

[Bhomesh]

what

• Converted EC2 instance configuration from individual variables to a structured object type
• Introduced new instance_configuration object to consolidate all EC2-related settings
• Created spot_configuration object to group spot instance specific parameters
• Updated resource configurations to use the new object structure
• Added default values and optional fields to maintain backward compatibility

why

• Improves code maintainability by grouping related configurations together
• Reduces the number of individual variables that need to be managed
• Makes it easier to validate input configurations with proper type constraints
• Simplifies module usage by providing a more structured and intuitive interface
• Follows HashiCorp's recommended practices for variable organization
• Makes it easier to extend the module with new instance configurations in the future