Downgrade resource_class or make it configurable in jobs

[stevenpitts]

Is your feature request related to a problem? Please describe.
circleci/heroku jobs are unnecessarily expensive. The jobs don't set a resource_class, so they default to the Large resource class. This results in high compute credit usage.
The jobs provided by this orb are practically entirely network-bound and use very little cpu/memory. They could easily run on a Small resource class.

Describe the solution you'd like
The orb's executor should have a resource_class field set to small.

Describe alternatives you've considered
The resource_class could be made configurable, such as in the circleci/aws-ecr orb, but there's really no point since no jobs in this orb are cpu or memory bound.
Alternatively, I could rewrite the entire orb so that I can specify the resource_class, but that doesn't feel like the right solution here.