fire experiment pixels

[SabrinaTardio]

Task/Issue URL: https://app.asana.com/0/1204186595873227/1208775176602791/f
Tech Design URL: https://app.asana.com/0/1204186595873227/1208682592686299
CC:

Description: Implements PixelExperimentKit to provide Experiment metrics API to clients and fires pixel for preset experiment metrics

Optional E2E tests:

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Steps to test this PR:

1. Somewhere in the code base (maybe in FeatureFlag) add this Experiment Feature Flag:

public struct CredentialsSavingFlag: FeatureFlagExperimentDescribing {
    public init() {}

    public typealias CohortType = Cohort

    public var rawValue = "credentialSaving"

    public var source: FeatureFlagSource = .remoteReleasable(.subfeature(AutofillSubfeature.credentialsSaving))

    public enum Cohort: String, FlagCohort {
        case control
        case blue
    }
}

2. Assign the cohort somewhere (for example in HomePageContinueSetUpModel initialiser)
   let cohort = Application.appDelegate.featureFlagger.getCohortIfEnabled(for: CredentialsSavingFlag())

3. In AppConfigurationURLProvider on line 57 change the url with https://www.jsonblob.com/api/jsonBlob/1313198249494110208

4. Run the app and check that the following pixel is sent: m_mac_experiment_enroll_credentialsSaving_blue (or control) with the expected parameters

5. Perform 4 searches and check that only one experiment pixel is fired: m_mac_experiment_metrics_credentialsSaving_blue ["pixelSource": "browser-dmg", "metric": "search", "enrollmentDate": "2024-12-04", "value": "1", "conversionWindowDays": "0”]

6. Move the system date to the following day and perform 4 searches and 4 site visits, only 2 pixels should be fired:
   m_mac_experiment_metrics_credentialsSaving_blue ["pixelSource": "browser-dmg", "metric": "search", "enrollmentDate": "2024-12-05", "value": "1", "conversionWindowDays": “1”]
   m_mac_experiment_metrics_credentialsSaving_blue ["pixelSource": "browser-dmg", "metric": “ app_use", "enrollmentDate": "2024-12-05", "value": "1", "conversionWindowDays": “1”]

7 Try the same up to day 4

8 On day 5 do the same and in this case you should see multiple pixels:
👾[Unique By Name And Parameters-Fired] m_mac_experiment_metrics_credentialsSaving_control ["conversionWindowDays": "5", "value": "1", "enrollmentDate": "2024-12-04", "metric": "search", "pixelSource": "browser-dmg"]
👾[Unique By Name And Parameters-Fired] m_mac_experiment_metrics_credentialsSaving_control ["pixelSource": "browser-dmg", "conversionWindowDays": "5-7", "enrollmentDate": "2024-12-04", "value": "1", "metric": "search"]
👾[Unique By Name And Parameters-Fired] m_mac_experiment_metrics_credentialsSaving_control ["pixelSource": "browser-dmg", "value": "1", "conversionWindowDays": "5", "metric": "app_use", "enrollmentDate": "2024-12-04"]
👾[Unique By Name And Parameters-Fired] m_mac_experiment_metrics_credentialsSaving_control ["metric": "app_use", "conversionWindowDays": "5-7", "pixelSource": "browser-dmg", "enrollmentDate": "2024-12-04", "value": "1"]
👾[Unique By Name And Parameters-Fired] m_mac_experiment_metrics_credentialsSaving_control ["value": "4", "pixelSource": "browser-dmg", "conversionWindowDays": "5-7", "metric": "search", "enrollmentDate": "2024-12-04"]
👾[Unique By Name And Parameters-Fired] m_mac_experiment_metrics_credentialsSaving_control ["value": "4", "enrollmentDate": "2024-12-04", "metric": "app_use", "pixelSource": "browser-dmg", "conversionWindowDays": "5-7”]

Note: I recommend Filter by Unique By Name And Parameters-Fired

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[jaceklyp]

LGTM :)