Restructure backend.tf to accomodate local development

Overview

There are obstacles to testing local development in personal AWS accounts. One is that the backend state for each account cannot use the same bucket name - requiring users to change the code manually before pushing changes to the repo.

In order to better facilitate local development for users using a personal AWS account, we need to provide a shell for the backend.tf code (instead of hardcoding it) and update the GitHub Action workflows to use a prod.backend.tfvars file.

Action Items

[x] Update the backend.tf file and replace w/ the following (we need it empty)
```
terraform {
backend "s3" {
}
}
```

[x] Create a prod.backend.tfvars file with the following configuration:

bucket         = "hfla-ops-terraform-state"
key            = "devops-security/terraform.tfstate"
region         = "us-west-2"
dynamodb_table = "hfla_ops_terraform_table"
encrypt        = true

[x] Add instructions to CONTRIBUTING.md for developers to create a backend.tfvars file w/ their backend configs

bucket         = "dev-specific-hfla-ops-terraform-state"
key            = "devops-security/terraform.tfstate"
region         = "us-west-2"
dynamodb_table = "dev-specific-hfla_ops_terraform_table"
encrypt        = true

[x] update the Terraform workflows (Terraform Plan Workflow / Terraform Apply Workflow) to add the below to the dflook blocks
```
with:
backend_config_file: prod.backend.tfvars
```
[x] add backend.tfvars to .gitignore
[x] push code changes and open a PR to test

Resources

https://github.com/dflook/terraform-plan#inputs

hackforla / devops-security