Support to separate step functions states into different yml files

This is a Feature Proposal

Description

For feature proposals:

• It is currently possible to create a separate state machine (#177) within a different file; however, it would be nice to also allow the possibility to split states into their own files as well, so that users do not have to re-define the same functionality multiple times. It does not seem like the plugin supports a similar approach at the state level, though.

I believe the definition may look something like this:

stepFunctions:
  stateMachines:
    mundaneTask:
      name: '${self:provider.stage}-something-repetitive  
      definition:
        Comment: 'Starts a file upload'
        StartAt: repetitiveStep
        States:
          repetitiveStep: ${file(something-repetitive.yml)}

Any feedback or help would be appreciated!

No source files, tests, or entry points are named in the proposal. Start by tracing how the plugin currently loads state machine definitions and handles ${file(...)} references, then determine the expected behavior for state-level files from the YAML example. Done should include documented support for separate state definitions and coverage for the resulting configuration.