Add script that build iOS frameworks build

[sunmou99]

Added ios build script (replaced the old one). The generated frameworks locate at dir "/ios_build" by default.
Added ios build workflow.

Tried to build different architectures in parallel, but it's more likely to break.

10/06/2020 update:
Build different architectures in parallel, build time reduced from ~33 minutes to ~20 minutes.

[vimanyu]

Looks very clean overall!
Super minor comments.

[paulb777]

It could be worth sync'ing up with the iCore team before going too far on developing scripts to generate frameworks. Apple is deprecating frameworks in favor of xcframeworks with the onset of Catalyst and arm64 macs.

It also might make sense to collaborate more on tooling since we already have some tooling for transforming Firebase sources to binaries at https://github.com/firebase/firebase-ios-sdk/tree/master/ZipBuilder

[sunmou99]

@paulb777 Thanks for calling out.
I talked with Gran, it seems that ZipBuilder is not the tool we could use here.
xcframeworks sounds good, but CMake doesn't support xcframeworks. Also, I don't know if xcframeworks will affect the test codes.
Who (from iCore) should I reach out to for more information?

The base branch was changed.