Create an executor parameter to allow for custom executors

[jenny-miggin]

Describe Request:

A static executor is currently defined in the check job, but this does not allow for any executor customisations. Allow for a custom executor to be used in place of this, if required

Examples:

Sample future config:

version: 2.1

orbs:
  shellcheck: circleci/[email protected]

executors:
  my-small-executor:
    docker:
      - image: cimg/base:stable
    resource_class: small

workflows:
  my_workflow:
    jobs:
      - shellcheck/check:
          dir: ./myScripts
          exclude: SC2148
          executor: my-small-executor

Supporting Documentation Links:

n/a