feat(executors): add `resource_class` overrides

[wyardley]

Checklist

• [ ] All new jobs, commands, executors, parameters have descriptions
• [ ] Examples have been added for any significant new features
• [ ] README has been updated, if necessary

Motivation, issues

In many orgs, the default is now to use large executors, which costs more money, and is almost always going to be overkill for typical uses of this orb.

Description

• Add ability to override the resource_class for the executor
• Default the executor resource_class to "small", which should be sufficient for most uses of this orb

[wyardley]

Open to changing the default or otherwise adjusting this

[wyardley]

Closed now that https://github.com/joinforma/circleci-orb-node/pull/101 is merged