Feature request: Supports build and combine each platform framework to XCFramework for Xcode 11

[dreampiggy]

• carthage install method: [x] homebrew
• which carthage: /usr/local/bin/carthage
• carthage version: 0.32.0
• xcodebuild -version: Xcode 11.0

Feature request

See WWDC Session: https://developer.apple.com/videos/play/wwdc2019/416/
Xcode Release Notes: New Features - XCFramework

Xcode 11 introduce a new bundled framework format: .xcframework

It can contains all of massive platform && architecture combination of current .framework.

For example, if your framework supports all Apple platform, you'll need 4 targets and generate 8 frameworks. If you support Static Framework, this number bumped to 16.

• iOS(armv7 arm64)
• iOS Simulator (x86_64)
• macOS (x86_64)
• macOS UIKit (x86_64)
• tvOS(arm64)
• tvOS Simulator (x86_64)
• watchOS (armv7k arm64_32)
• watchOS SImulator (x86_64)

Then when you have 8 frameworks, you need to open Finder and drag them one by one for your Application target. You should also be careful not to mix them up (such as drag a iOS framework into WatchKitExtention target).

It's cool if Carthage can automatically generate one XXX.xcframework which combine all the individual framework for all valid target, then let user just drag one is OK.

Note the xcframework can also be dependented, so it's also cool for framework author, to just dependency one Carthage/build/YYY.xcframework in Xcode build setting.

[tmspzz]

Duplicate of #2799