Github action to validate Flipper and FlipperKit pod (#1026)

Summary:
This PR adds Github Action to validate the podspec file. Validation is crucial, if it podspecs don't validate then we won't be able publish pods to the trunk.
## Changelog

Add Github Action to validate FlipperKit and Flipper pod
Pull Request resolved: https://github.com/facebook/flipper/pull/1026

Test Plan: Github Action passes.

Reviewed By: mweststrate

Differential Revision: D21130523

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 0116120b9bbb073516798d4d7f08b347ed9755a3

.github/workflows/iOS-pod-lint.yml

@@ -0,0 +1,26 @@
+name: Validate Podspecs
+on: [push, pull_request]
+
+jobs:
+  lint-flipperkit_pod:
+    runs-on: macos-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install Dependences
+      run: pod repo update
+      
+    - name: Lint FlipperKit
+      run: pod spec lint FlipperKit.podspec --use-libraries --allow-warnings --verbose --skip-import-validation
+    
+  lint-flipper_pod:
+    runs-on: macos-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install Dependences
+      run: pod repo update
+    - name: Lint Flipper
+      run: pod spec lint Flipper.podspec --use-libraries --allow-warnings --verbose
+      
+