feat: Add support for GitHub Enterprise Cloud

[unfunco]

Adds support for GitHub Enterprise Cloud customers by adding a new variable called enterprise_slug which when populated, will configure the OIDC provider with a different issuer URL, and will condition the role to be assumed only when the iss claim includes the enterprise slug.

[bridgecrew]

Bridgecrew has found errors in this PR ⬇️

[bridgecrew]

  Ensure IAM policies do not allow data exfiltration
    Resource: aws_iam_policy_document.example | Bridgecrew ID: BC_AWS_IAM_55 | Checkov ID: CKV_AWS_108

How to Fix

        data "aws_iam_policy_document" "example" {
              statement {
                sid = "1"
                effect = "Allow"
                actions = [
                    "lambda:CreateFunction",
                    "lambda:CreateEventSourceMapping",
                    "dynamodb:CreateTable",
                ]
                resources = [
                  "*",
                ]
              }
            }

Description

Data Exfiltration actions allow certain read-only IAM actions without resource constraints, such as s3:GetObject, ssm:GetParameter*, or secretsmanager:GetSecretValue.

1 - Unrestricted s3:GetObject permissions has a long history of customer data leaks
2 - ssm:GetParameter* and secretsmanager:GetSecretValue are both used to access secrets.
3 - rds:CopyDBSnapshot and rds:CreateDBSnapshot can be used to exfiltrate RDS database contents.

For more information, visit the cloudsplaining documentation
https://cloudsplaining.readthedocs.io/en/latest/glossary/data-exfiltration/

     🎉   Fixed by commit 618feaa - Update example to avoid a security warning