feat: Add include_default_tag variable

[joshuamkite]

Description

Add include_default_tag boolean variable to control whether the default terraform-aws-modules = "apigateway-v2" tag is included in resources.

Based on the include_default_tag variable from the terraform-aws-modules/lambda module.

Changes

• Add include_default_tag variable (default: true)
• Update tag merge logic in main.tf
• Add test case in examples/complete-http/main.tf
• Update terraform-docs in README

Usage

module "api_gateway" {
  source = "terraform-aws-modules/apigateway-v2/aws"
  
  include_default_tag = false
  
  tags = {
    Environment = "production"
  }
}

Backward compatible - defaults to true to maintain existing behavior.