feat: Add the capability to disable alert manager creation

[FlawlessByte]

Description

This PR adds a new create_alert_manager variable to the AWS Managed Service for Prometheus (AMP) Terraform module, allowing users to conditionally disable Alert Manager creation. The variable defaults to true to maintain backward compatibility.

Motivation and Context

• Resolves Add option to disable Alert Manager creation in AWS Managed Prometheus module #19

• Currently, the module always provisions an Alert Manager along with the AMP workspace

• Users with existing external alerting systems or those who don't want Alert Manager are forced to create unnecessary resources

• This adds extra overhead and confusion for users who don't need Alert Manager functionality

Breaking Changes

None. This change is fully backward compatible:

• The create_alert_manager variable defaults to true, preserving existing behavior
• All existing configurations will continue to work without modification
• No existing resources or configurations are affected

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
  • Updated examples/complete/main.tf to include create_alert_manager = true in the main example
• I have tested and validated these changes using one or more of the provided examples/* projects
  • Verified that the module works correctly with both create_alert_manager = true (default) and create_alert_manager = false
  • Confirmed that Alert Manager resources are only created when both create and create_alert_manager are true
  • Tested that the module maintains backward compatibility with existing configurations
• I have executed pre-commit run -a on my pull request
  • All pre-commit hooks passed successfully
  • Documentation was auto-generated using terraform-docs
  • Code formatting and validation completed without issues

[antonbabenko]

This PR is included in version 3.1.0 🎉

[github-actions]

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