Remove "static_framework" from open-source pods

[SebastianSingle99]

Some podspec files like FirebaseCore.podspec or FirebaseMessaging.podspec have set s.static_framework = true, although they are now open sourced and could be used as dynamic framework (use_frameworks!).
Cocoapods says:
[!] The 'Pods-xxx target has transitive dependencies that include static frameworks: (FirebaseMessaging)

Setting static_framework to false works as expected. Could static_framework be removed?
Then each developer can decide if he wants to use dymanic frameworks or use the newer use_modular_headers-feature to build static libraries.

[paulb777]

@SebastianSingle99 Thanks for the suggestion!

There are a few bugs with CocoaPods 1.5.x that prevent this, but it should be doable with CocoaPods 1.6.x. Also it would be a breaking change for Firebase.

I've added it to the next major release milestone for deeper investigation.

[paulb777]

My biggest concern about this change would be causing some developers to have several more dynamic frameworks along with a potential performance impact.

We might want to wait for this until CocoaPods has the capability to pick and choose which pods are built dynamically and statically.

[paulb777]

This is a bigger change than we want to make for the default Firebase use case.

Likely we'll wait for better CocoaPods support.

If anyone wants this in the meantime, they can use modified podspec version.

[paulb777]

Reconsider for Firebase 7 since CocoaPods 1.9.0 adds the ability for Podfile's to specify static or dynamic at an individual pod granularity.

[paulb777]

CocoaPods 1.9.0 only added more flexibility on the use_frameworks! directive and pushed out the proposal to provide granularity at the individual pod.