Add support for composite actions (#514)

* Add support for composite actions

* Fix to make more complex composite actions work

* Fix to make more complex composite actions work

* Let's validate the steps in the composite steps to fail on uses and run's without shell, like the real world

* Add support for composite actions

* Add workflow to test composite actions

* Log instead of panicing when output is mismatched

* Merge maps so environment variables are not lost

* Remove Debug

* Correect merge error

* Remove invalid composite tests.

* Fix composite test

Co-authored-by: Casey Lee <cplee@nektos.com>
Co-authored-by: monkers <mikem@msquaredconsulting.co.uk>
Co-authored-by: Mike Moncrieffe <69815687+mikemonkers@users.noreply.github.com>

pkg/runner/testdata/uses-composite/push.yml

@@ -0,0 +1,18 @@
+name: uses-docker-url
+on: push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: ./uses-composite/composite_action
+      id: composite
+      with:
+        test_input_required: 'test_input_required_value'
+        test_input_optional: 'test_input_optional_value'
+
+    - if: steps.composite.outputs.test_output != "test_output_value"
+      run: |
+        echo "steps.composite.outputs.test_output=${{ steps.composite.outputs.test_output }}"
+        exit 1
+