Flag to not override existing tags

[eranelbaz]

Is your feature request related to a problem? Please describe.
When using Terratag with existing tags Terratag override my tags
See example
main.tf -

resource "aws_s3_bucket" "example" {
  bucket = "my-tf-test-bucket"

  tags = {
    Name        = "My bucket"
    Environment = "Dev"
  }
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
}

provider "aws" {
  region = "us-east-1"
}

Terratag command -

> /opt/terratag -tags="{\"Name\":\"By Terratag\"}" -rename=false 

Results with this file

resource "aws_s3_bucket" "example" {
  bucket = "my-tf-test-bucket"

  tags = merge({
    "Name"        = "My bucket"
    "Environment" = "Dev"
  }, local.terratag_added_main)
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
}

provider "aws" {
  region = "us-east-1"
}

locals {
  terratag_added_main = {"Name"="By Terratag"}
}

Plan results in

      + tags                        = {
          + "Environment"         = "Dev"
          + "Name"                = "By Terratag"
        }
      + tags_all                    = {
          + "Environment"         = "Dev"
          + "Name"                = "By Terratag"
        }

Describe the solution you'd like
Flag that enables a different behaviour of merging tags instead of overriding tags, meaning the plan will result in

      + tags                        = {
          + "Environment"         = "Dev"
          + "Name"                = "My bucket"
        }
      + tags_all                    = {
          + "Environment"         = "Dev"
          + "Name"                = "My bucket"
        }

As Name exists in our Terraform configuration

Describe alternatives you've considered

Additional context