Add configurable OS versions of Xcode Simulators.

[descorp]

Description

I want to run my tests on different versions of iOS / macOS / watchOS using the latest Xcode.

Right now, each version of Xcode comes with the corresponding "build-in" versions of SDKs.
So, the only way to run an iPhone Simulator with an iOS 12.4 is by switching to an Xcode 10.3.

The problem is that it is not possible to run swift5 code on Xcode 10.3.

Possible solution

1. Stash all .sdk images from all Xcodes /Applications/Xcode_[Version].app/Contents/Developer/Platforms/[Platform]/Developer/SDKs/ in one place.

2. Create a configuration for environment like:

   env:
      DEVELOPER_DIR: '/Applications/Xcode_11.4.1.app/Contents/Developer'
      DEVELOPER_SDK: ['iPhoneSimulator13.2', 'WatchOS6.1' ]
   

3. Move / Copy .sdk to a correspondent DEVELOPER_DIR.

Virtual environments affected

• macOS 10.15

Can this tool be installed during the build?
Yes

[maxim-lobanov]

Duplicate of #551.
We have a plan to pre-install more simulators but can't provide any ETA for now.
I am closing this one as duplicate so you can track status of this request in #551