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
