Add a developer option for history sharing on invite

Adds a new developer flag, enabling our experimental support for MSC4268.

ElementX/Sources/Screens/Settings/DeveloperOptionsScreen/DeveloperOptionsScreenModels.swift

@@ -42,6 +42,7 @@ protocol DeveloperOptionsProtocol: AnyObject {
     var hideUnreadMessagesBadge: Bool { get set }
     var fuzzyRoomListSearchEnabled: Bool { get set }
     var enableOnlySignedDeviceIsolationMode: Bool { get set }
+    var enableKeyShareOnInvite: Bool { get set }
     var elementCallBaseURLOverride: URL? { get set }
     var knockingEnabled: Bool { get set }
     var threadsEnabled: Bool { get set }

ElementX/Sources/Screens/Settings/DeveloperOptionsScreen/View/DeveloperOptionsScreen.swift

@@ -75,6 +75,16 @@ struct DeveloperOptionsScreen: View {
                 Text("This setting controls how end-to-end encryption (E2EE) keys are exchanged. Enabling it will prevent the inclusion of devices that have not been explicitly verified by their owners.")
             }
 
+            Section {
+                Toggle(isOn: $context.enableKeyShareOnInvite) {
+                    Text("Share encrypted history with new members")
+                    Text("Requires app reboot")
+                }
+            } footer: {
+                Text("When inviting a user to an encrypted room that has history visibility set to \"shared\", share encrypted history with that user, and accept encrypted history when you are invited to such a room.")
+                Text("WARNING: this feature is EXPERIMENTAL and not all security precautions are implemented. Do not enable on production accounts.")
+            }
+
             Section {
                 TextField("Leave empty to use EC locally", text: $elementCallURLOverrideString)
                     .autocorrectionDisabled(true)