Implements setup-workspace GitHub Action + caching

[kerberjg]

Status: WIP + blocked by #171 (needs rebase)

---

This feature allows workspace-automation to be run in any GHA environment using the setup-workspace action.
For example, when building an app/plugin using ivi-homescreen, you can just import this action into your CI as follows:

jobs:
  ...
  steps:
    - name: Create workspace
      uses: meta-flutter/workspace-automation/setup-workspace@v2.0
      
    - name: Run
      run: |
          source ./setup_env.sh
          ls -la $FLUTTER_WORKSPACE
          dart --version
          echo "Hello world from Flutter Workspace!"

Benefits include:

• ~30-60% speedup vs raw run
• Compatible with Ubuntu/Fedora (tested against our build matrix)
• Caching (apt/dnf/dart-pub/pip/artifacts/git/flutter-sdk)
  • (optional!) whole workspace, including build files/configs via cache-whole
  • Auto/manual cache key

[kerberjg]

ubuntu-legacy build currently failing due to #171 and meta-flutter/llvm-toolchains#1 being incomplete

[jwinarske]

@kerberjg needs a rebase

[kerberjg]

@jwinarske done