add github actions workflow

.github/workflows/gradle.yml

@@ -0,0 +1,27 @@
+# This workflow uses actions that are not certified by GitHub.
+# They are provided by a third-party and are governed by
+# separate terms of service, privacy policy, and support
+# documentation.
+# This workflow will build a Java project with Gradle and cache/restore any dependencies to improve the workflow execution time
+# For more information see: https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-java-with-gradle
+
+name: Java CI with Gradle
+
+on:
+  push:
+    branches: [ "master" ]
+
+jobs:
+  build-gradle-project:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout project sources
+      uses: actions/checkout@v3
+    - name: Setup Gradle
+      uses: gradle/gradle-build-action@v2
+    - name: Run build with Gradle Wrapper
+      run: ./gradlew shadowjar
+    - uses: actions/upload-artifact@v4
+      with:
+        name: Archipelago.MultiClient.Java-SNAPSHOT
+        path: build/libs/Archcipelago.MultiClient.Java-*-all.jar