Add CI workflow for Swift Package Manager (#8)

Added a GitHub workflow to build and run SPM test targets.

.github/workflows/spm.yml

@@ -0,0 +1,27 @@
+name: spm
+
+on:
+  pull_request:
+  schedule:
+    # Run every day at 11pm (PST) - cron uses UTC times
+    - cron:  '0 7 * * *'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  swift-build-run:
+    runs-on: macOS-latest
+    strategy:
+      matrix:
+        target: [iOS, tvOS, macOS, catalyst]
+    steps:
+    - uses: actions/checkout@v3
+    - uses: ruby/setup-ruby@v1
+    - name: Setup Scripts Directory
+      run: ./setup-scripts.sh
+    - name: Initialize xcodebuild
+      run: xcodebuild -list
+    - name: iOS Unit Tests
+      run: scripts/third_party/travis/retry.sh scripts/build.sh AppCheck ${{ matrix.target }} spm

.ruby-version

@@ -0,0 +1 @@
+ruby-2.7

setup-scripts.sh

@@ -20,3 +20,5 @@ cd firebase-ios-sdk
 git checkout master -- scripts
 cd ..
 ln -s firebase-ios-sdk/scripts scripts
+
+gem install xcpretty