Multifeature support in Desktop and Mobile including Legacy Desktop

[jaredlockhart]

Because

• We need to support legacy experiments in all clients using the single feature schema
• We need to support legacy clients in desktop that encounter a multifeature schema but expect the feature key to exist
• We need to support future mobile clients that expect only the features key to be present

This commit

• Adds a SingleFeatureBranch for all legacy clients
• Adds a MultiFeatureDesktopBranch which allows new desktop experiments to use multifeature keys without breaking legacy clients
• Adds a MultiFeatureMobileBranch which allows new mobile experiments to use multifeature keys
• Adds sample experiments for each of desktop/android/ios using each of their respective schemas

[heres-maria-sv-inactive]

We have completed testing on the test recipes provided. You can find the TestRail test runs here. We have found the following issue, which we understand will be resolved after Bug 1738286 is implemented:

• Bug 1745182 The upgradeDialog feature is not disabled in the Control branch of a Nimbus experiment

Desktop Version/Platforms Tested:

• Firefox Release 95 (Build ID: 20211129150630) on Windows 10, Linux Mint 20.1 and macOS 11.6.

Mobile version/Platform Tested:

• Android: Firefox Beta null (built from the release_v96.0.0 branch as Beta build) (AS v86.2.1) (Build #2015850483, 3c5b9f496+) (AC: 96.0.0, ea5bd2687c) (GV: 96.0-20211206194555) on Android 9 - Samsung Galaxy S8.
• iOS: Firefox Daylight 39.0 (Fennec) on the Xcode Simulator (iOS 15, iPhone 13 Pro)