Support pre and post steps in Composite Actions

Describe the enhancement

Currently, three Action types are supported: Docker, JavaScript and Composite (see https://docs.github.com/en/actions/creating-actions/about-custom-actions#types-of-actions). However, features pre, pref-if, post and post-if are only is supported in JavaScript Actions only (see https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#runs-for-javascript-actions). Therefore, users writing workflows/actions using some scripting language such as Python are forced to wrap the steps in JavaScript in order to register pre and post steps. See, for example, https://github.com/pyTooling/Actions/tree/main/with-post-step:

name: With post step
description: 'Generic JS Action to execute a main command and set a command in a post step.'
inputs:
  main:
    description: 'Main command/script.'
    required: true
  post:
    description: 'Post command/script.'
    required: true
  key:
    description: 'Name of the state variable used to detect the post step.'
    required: false
    default: POST
runs:
  using: 'node12'
  main: 'main.js'
  post: 'main.js'

const { exec } = require('child_process');

function run(cmd) {
  exec(cmd, (error, stdout, stderr) => {
    if ( stdout.length != 0 ) { console.log(`${stdout}`); }
    if ( stderr.length != 0 ) { console.error(`${stderr}`); }
    if (error) {
      process.exitCode = error.code;
      console.error(`${error}`);
    }
  });
}

const key = process.env.INPUT_KEY.toUpperCase();

if ( process.env[`STATE_${key}`] != undefined ) { // Are we in the 'post' step?
  run(process.env.INPUT_POST);
} else { // Otherwise, this is the main step
  console.log(`::save-state name=${key}::true`);
  run(process.env.INPUT_MAIN);
}

The complexity might be simplified if Python or Bash or PowerShell were supported similarly to JavaScript:

name: 'Login to container registries and set a post step'

runs:
  using: 'python'
  main: 'precmd.py'
  post: 'postcmd.py'

Additional information

Alternatively, since regular steps support Python as a built-in shell already, the same capability might be achieved if Composite Actions supported fields pre and post as a complement to steps. For instance:

name: 'Login to container registries and set a post step'

inputs:
  precmd:
    description: 'Pre command'
    required: true
  postcmd:
    description: 'Pre command'
    required: true

runs:
  using: 'composite'
  pre:
    - shell: python
      run: ${{ inputs.precmd }}
  post:
    - shell: python
      run: ${{ inputs.postcmd }}
  steps:
    - ...

/cc @thboop, per https://github.com/actions/runner/issues/646#issuecomment-901336347

Start by reviewing the runner's existing Composite Action handling and the linked discussion around issue 646. Compare the requested pre, post, pre-if, and post-if behavior with current JavaScript Action support, then define and test the intended lifecycle semantics for composite steps and supported shells.