Add instance parameter to apply_load_aws script.

[dmkozh]

Description

Add instance parameter to apply_load_aws script for the sake of customization.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

This PR adds an --instance-type CLI parameter to the aws-init flow of the apply-load AWS orchestration script, allowing callers to customize the EC2 instance type used for apply-load runs while preserving the prior default.

Changes:

• Replace the hard-coded INSTANCE_TYPE constant with DEFAULT_INSTANCE_TYPE.
• Thread an instance_type parameter through aws_init() into start_ec2_instance().
• Add --instance-type to the aws-init subcommand with a default of c5d.2xlarge.