[AWS Batch] Add instance_types config option for AWS Batch EC2/SPOT environments

[bystepii]

Currently instanceTypes is hardcoded to ['optimal'] for EC2/SPOT compute environments. This adds an optional instance_types config key that accepts a list of instance types (e.g. ['m5.large', 'c5.xlarge'], ['m5'], or ['optimal']). Defaults to ['optimal'] when omitted, preserving backward compatibility.

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