feat(engine): More input validation

[daryllimyt]

Features

• Validate INPUTS context
  • Check that any INPUTS context invocation matches what's been defined in the inputs top level key
  • Ensure that inputs don't contain templates, as these are static inputs.
• Adjust DSLInput.entrypoint API
  • Add entrypoint.expects to allow the user to define a schema to which the trigger input must conform
  • Move entrypoint to entrypoint.ref

e.g. old

entrypoint: my_action

new

entrypoint:
    ref: my_action
    expects: <WIP>
    ...

Changes

• Update all playbooks to reflect new entrypoint top level key shape

Tests

• All unit tests passing