Add github action for testing Sample app (#1025)

Summary:
This PR adds Github actions to verify that the Sample Objective-C, Swift and Tutorial app build successfully.

## Changelog

Add Github Actions for building Sample Objective-C, Swift and Tutorial app.
Pull Request resolved: https://github.com/facebook/flipper/pull/1025

Test Plan: Github Action passes successfully.

Reviewed By: jknoxville

Differential Revision: D21098106

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 6edd5a3cba74b5ea843abba366c4ad9dc550cf4e

.github/workflows/iOS-Sample.yml

@@ -0,0 +1,48 @@
+name: Build iOS apps
+on: [push, pull_request]    
+jobs:       
+  build-sample-objc:
+    runs-on: macos-latest
+    defaults:
+      run:
+        working-directory: iOS/Sample
+        
+    steps:
+    - uses: actions/checkout@v2      
+    - name: Install dependencies
+      run: pod install --repo-update
+    - name: Build Sample app
+      run: |
+        IPHONESIMULATOR=$(xcodebuild -showsdks | grep -o "iphonesimulator[0-9]\\+\\.[0-9]\\+$") && echo $IPHONESIMULATOR
+        xcodebuild build -workspace Sample.xcworkspace -scheme Sample -sdk $IPHONESIMULATOR
+        
+  build-sample-swift:
+    runs-on: macos-latest
+    defaults:
+      run:
+        working-directory: iOS/SampleSwift
+        
+    steps:
+    - uses: actions/checkout@v2      
+    - name: Install dependencies
+      run: pod install --repo-update
+    - name: Build SampleSwift app
+      run: |
+        IPHONESIMULATOR=$(xcodebuild -showsdks | grep -o "iphonesimulator[0-9]\\+\\.[0-9]\\+$") && echo $IPHONESIMULATOR
+        xcodebuild build -workspace SampleSwift.xcworkspace -scheme SampleSwift -sdk $IPHONESIMULATOR
+        
+  build-tutorial:
+    runs-on: macos-latest
+    defaults:
+      run:
+        working-directory: iOS/Tutorial
+        
+    steps:
+    - uses: actions/checkout@v2      
+    - name: Install dependencies
+      run: pod install --repo-update
+    - name: Build Tutorial app
+      run: |
+        IPHONESIMULATOR=$(xcodebuild -showsdks | grep -o "iphonesimulator[0-9]\\+\\.[0-9]\\+$") && echo $IPHONESIMULATOR
+        xcodebuild build -workspace Tutorial.xcworkspace -scheme Tutorial -sdk $IPHONESIMULATOR
+