feat: Cocoapods support (#4667)

Dockerfile

lib/datasource/pod/index.spec.ts

lib/datasource/pod/index.ts

lib/manager/cocoapods/__fixtures__/Podfile.complex

lib/manager/cocoapods/__fixtures__/Podfile.simple

lib/manager/cocoapods/__snapshots__/artifacts.spec.ts.snap

lib/manager/cocoapods/__snapshots__/extract.spec.ts.snap

lib/manager/cocoapods/artifacts.spec.ts

lib/manager/cocoapods/artifacts.ts

lib/manager/cocoapods/extract.spec.ts

lib/manager/cocoapods/extract.ts

lib/manager/cocoapods/index.ts

lib/manager/cocoapods/readme.md

lib/manager/cocoapods/update.spec.ts

lib/manager/cocoapods/update.ts