Example / support for custom pod spec repositories

[jerrymarino]

Cocoapods supports “spec repo”s to share and or customize specs to many repos. Under cocoapods, a user may run pod install to find than it needs to be re-ran again with —repo-update to pull spec repos.

Support this feature to better support multi-repos: where it doesn’t make sense to put podspecs in a single repo. A spec repo might be expressed as an external workspace dependency that provides specs for a version. Then, the user can pin to a specific spec repo; Bazel will manage updating the spec repo without having to re-run pod install. This is distinct from a Bazel mod registry or repo; a spec repo provides pod specifications only