Initial conversion to use of composite GH actions

Author: gwynne

action.yml

@@ -0,0 +1,39 @@
+name: 'Vapor CI Support Tools'
+description: 'Provides common support and beahvior for Vapor CI workflows via the new GitHub Actions support for "composite" actions.'
+inputs:
+  command:
+    description: 'Which support tooling subfunction to execute.'
+    required: true
+outputs:
+  unit-test-swift-versions:
+    description: 'The set of Swift version specifiers to run unit tests against.'
+    value: ${{ steps.unit-test-swift-versions-impl.outputs.data }}
+  unit-test-swift-runners:
+    description: 'The set of Swift runners (underlying OS) to run unit tests on.'
+    value: ${{ steps.unit-test-swift-runners-impl.outputs.data }}
+  integration-test-swift-versions:
+    description: 'The set of Swift version specifiers to run integration tests against.'
+    value: ${{ steps.integration-test-swift-versions-impl.outputs.data }}
+  integration-test-swift-runners:
+    description: 'The set of Swift runners (underlying OS) to run integration tests on.'
+    value: ${{ steps.integration-test-swift-runners-impl.outputs.data }}
+runs:
+  using: "composite"
+  steps:
+    - id: unit-test-swift-versions-impl
+      run: echo "::set-output name=data::UNIMPLEMENTED"
+      shell: bash
+      if: ${{ inputs.command == 'unit-test-swift-versions' }}
+    - id: unit-test-swift-runners-impl
+      run: echo "::set-output name=data::UNIMPLEMENTED"
+      shell: bash
+      if: ${{ inputs.command == 'unit-test-swift-runners' }}
+    - id: integration-test-swift-versions-impl
+      run: echo "::set-output name=data::UNIMPLEMENTED"
+      shell: bash
+      if: ${{ inputs.command == 'integration-test-swift-versions' }}
+    - id: integration-test-swift-runners-impl
+      run: echo "::set-output name=data::UNIMPLEMENTED"
+      shell: bash
+      if: ${{ inputs.command == 'integration-test-swift-runners' }}
+