Seamless account switching feature flag (#1188)

Please review the release process for BrowserServicesKit
[here](https://app.asana.com/0/1200194497630846/1200837094583426).

**Required**:

Task/Issue URL:
https://app.asana.com/0/1142021229838617/1209000370243820/f
iOS PR: duckduckgo/iOS#3867
macOS PR: duckduckgo/macos-browser#3779
What kind of version bump will this require?: Major

**Description**:

Just a feature flag. Please see platform PRs for testing.

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**OS Testing**:

* [ ] iOS 14
* [ ] iOS 15
* [ ] iOS 16
* [ ] macOS 10.15
* [ ] macOS 11
* [ ] macOS 12

---
###### Internal references:
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)

Sources/BrowserServicesKit/PrivacyConfig/Features/PrivacyFeature.swift

@@ -162,6 +162,7 @@ public enum SyncSubfeature: String, PrivacySubfeature {
     case level1AllowDataSyncing
     case level2AllowSetupFlows
     case level3AllowCreateAccount
+    case seamlessAccountSwitching
 }
 
 public enum AutoconsentSubfeature: String, PrivacySubfeature {