Registered step functions can be referenced from a step definition

Changes

This MR is a continuation on from:

• Support running functions as steps (!301 - merged),
• Convert the script step to run as a step function (!303 - merged), and
• Temporarily disable runner e2e tests (!305 - merged).

Notably, this MR:

• Adds e2e tests back for the script step
• Adds a CHANGELOG entry for !303 (merged)
• Removes unused code that deployed the Runner in the pipeline
• Adds support for builtin://script so that custom functions can be referenced and run