ci: add GHA workflow for smithy-dotnet tests (#22)

Author: alex-chew

.github/workflows/smithy-dotnet.yml

@@ -0,0 +1,18 @@
+name: smithy-dotnet workflows
+
+on: ['push']
+
+jobs:
+  gradle-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          distribution: 'corretto'
+          java-version: '16'
+      - name: Execute tests
+        uses: gradle/gradle-build-action@v2
+        with:
+          arguments: test
+          build-root-directory: smithy-dotnet