add support for spot pricing

[cpolanec]

Launch Templates support spot pricing which can reduce the runtime costs of the EC2 instances compared to the current on-demand approach. Spot pricing requests introduce additional complexity when launched from CloudFormation. Deletion of a CloudFormation stack will not include the Spot Request. These lingering Spot Requests will continue to launch EC2 instances after the stack has been deleted. Additional work is necessary to remove these Spot Requests via the CLI in the Makefile and possibly the GitHub actions. Still, the possibility remains that Spot Requests could remain so it might be useful to have background jobs monitoring for them.