Request: Please support self hosted runners as Executors

[robotpistol]

Is there an existing issue that is already proposing this?

• [X] I have searched the existing issues

Is your feature request related to a problem? Please describe it

We use self hosted runners in our setup but to use them we need to set resource_class to an image name like cimg/go or whatever. Right now executors in the SDK are required to be and enumerated type (small, medium, large, etc.)

https://circleci.com/docs/resource-class-overview/#introduction

Describe the solution you'd like

It would be great to have a SelfHostedExecutor type that we could leverage.

my-self-hosted-executor:
    docker:
      - image: cimg/go:1.20
    resource_class: mynamespace/myrunner

Teachability, documentation, adoption, migration strategy

Just adding a new executor type should do. But it will require modifying the AnyResourceClass type to be able to support <namespace>/<runner> strings

What is the motivation / use case for changing the behavior?

As of right now There is no good way to represent this in code. My other option is to roll it into an orb and use it that way but it seems like overkill for this case.

[KyleTryon]

We can definitely look into this 👍 . Glad to see interest in this!