implement experiment manager

[SabrinaTardio]

Please review the release process for BrowserServicesKit here.

Required:

Task/Issue URL: https://app.asana.com/0/1204186595873227/1208687542689224/f
iOS PR: duckduckgo/macos-browser#3531
macOS PR: duckduckgo/macos-browser#3531
What kind of version bump will this require?: Minor

Optional:

Tech Design URL: https://app.asana.com/0/481882893211075/1208680035673275/f
CC:

Description: Implements the ExperimentCohortManager according to the TechDesign

Steps to test this PR:

1. Check tests are green
2. No PrivacyConfig Errors in the clients

OS Testing:

• iOS 14
• iOS 15
• iOS 16
• macOS 10.15
• macOS 11
• macOS 12

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Internal references:

Software Engineering Expectations
Technical Design Template

[jaceklyp]

Hey @SabrinaTardio, Before I start actually reviewing it, one thing that stands out to me is unnecessarily coupling with UserDefaults - hence we need to mock them in unit tests. Can we instead just protocolize Storing, and so the implementation of storing uses UserDefaults for real life cases and in tests we use in-memory storage. Please see the implementation of UserBehaviorMonitor + UserBehaviorMonitorTests in BSK or BrokenSitePromptLimiter + BrokenSitePromptLimiterTests.

[SabrinaTardio]

Hey @SabrinaTardio, Before I start actually reviewing it, one thing that stands out to me is unnecessarily coupling with UserDefaults - hence we need to mock them in unit tests. Can we instead just protocolize Storing, and so the implementation of storing uses UserDefaults for real life cases and in tests we use in-memory storage. Please see the implementation of UserBehaviorMonitor + UserBehaviorMonitorTests in BSK or BrokenSitePromptLimiter + BrokenSitePromptLimiterTests.

I thought about it but seemed a bit of an overkill since we can still test but happy to add a Storing class… I was in two minds anyway

[jaceklyp]

LGTM