chore(batch): init base EC2 class for ECS and EKS

[phuhung273]

Issue # (if applicable)

Close #35859

Reason for this change

• Keep codebase clean
• Prevent the case when adding a new feature to ECS but forgot to do the same for EKS
• Streamline future change, eg: (aws-batch): AWS Batch Compute Environment For AL2023 #35839
• A foundation to also reduce duplication for defaultInstanceClasses, useOptimalInstanceClasses, allocationStrategy, spotBidPercentage, instanceRole, launchTemplate, minvCpus, placementGroup and their related functions. I don't want to submit a huge refactor PR directly.

Description of changes

Init base EC2 class for ECS and EKS, start with only minimal things:

• property instanceTypes
• property instanceClasses
• method addInstanceType
• method addInstanceClass

Description of how you validated changes

Unit + Integ

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license