Add AlmaLinux 8 custom AMI support

[iamh2o]

Summary

• add explicit AlmaLinux 8 providers for environment, platform, shared, and Slurm resources
• apply existing Enterprise Linux 8 package, Lustre, networking, and scheduler behavior to AlmaLinux 8
• explicitly skip EFA installation because the current EFA installer does not support AlmaLinux 8
• add generic ChefSpec, Kitchen, CloudWatch, and style coverage

Scope

This supports customer-provided AlmaLinux 8 parent images. It does not claim that AWS ParallelCluster publishes an AlmaLinux image.

Tracks aws/aws-parallelcluster#7482.

Companion CLI PR: aws/aws-parallelcluster#7483

Tests

• 1,975 edited-surface ChefSpec examples passed
• 19 CloudWatch Python tests passed
• EFA skip behavior passed focused ChefSpec coverage
• 66 changed Ruby files passed Cookstyle
• changed Ruby and Python files passed syntax checks
• an ImageBuilder build from an official AlmaLinux 8 x86_64 parent completed, and the resulting custom AMI passed the ImageBuilder boot/test phase