Add .enable option for feature flag (#1223)

**Required**:

Task/Issue URL:  https://app.asana.com/0/0/1209364252386095/f

iOS PR:  duckduckgo/iOS#3952
macOS PR: duckduckgo/macos-browser#3854
What kind of version bump will this require?: Minor

**Optional**:

Tech Design URL:
CC:

**Description**:
Adds a new .enable option for Feature Flags which doesn’t require
privacy config checks.
Counterpart of “.disable"

Sources/BrowserServicesKit/FeatureFlagger/FeatureFlagger.swift

@@ -148,6 +148,9 @@ public enum FeatureFlagSource {
     /// Completely disabled in all configurations
     case disabled
 
+    /// Completely enabled in all configurations
+    case enabled
+
     /// Enabled for internal users only. Cannot be toggled remotely
     case internalOnly((any FeatureFlagCohortDescribing)? = nil)
 
@@ -333,6 +336,8 @@ public class DefaultFeatureFlagger: FeatureFlagger {
         switch featureFlag.source {
         case .disabled:
             return false
+        case .enabled:
+            return true
         case .internalOnly:
             return internalUserDecider.isInternalUser
         case .remoteDevelopment(let featureType):

Tests/BrowserServicesKitTests/FeatureFlagging/DefaultFeatureFlaggerTests.swift

@@ -89,6 +89,11 @@ final class DefaultFeatureFlaggerTests: XCTestCase {
         XCTAssertFalse(featureFlagger.isFeatureOn(for: FeatureFlagSource.disabled))
     }
 
+    func testWhenEnabled_sourceEnabled_returnsTrue() {
+        let featureFlagger = createFeatureFlagger()
+        XCTAssertTrue(featureFlagger.isFeatureOn(for: FeatureFlagSource.enabled))
+    }
+
     func testWhenInternalOnly_returnsIsInternalUserValue() {
         let featureFlagger = createFeatureFlagger()
         internalUserDeciderStore.isInternalUser = false