[Nix] Add cocoapods for ios build

Summary: Add cocoapods to allow for pod installs

Test Plan:
Install XCode

nix develop
cd native/ios && pod install && xcodebuild -workspace Comm.xcworkspace -scheme Comm -destination generic/platform=iOS -allowProvisioningUpdates

Reviewers: atul, ashoat, varun, marcinwasowicz

Reviewed By: atul, ashoat

Subscribers: palys-swm, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4480