Add a developer option for history sharing on invite (#4821)

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

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -112,6 +112,17 @@ enum class FeatureFlags(
         defaultValue = { false },
         isFinished = false,
     ),
+    EnableKeyShareOnInvite(
+        key = "feature.enableKeyShareOnInvite",
+        title = "Share encrypted history with new members",
+        description = "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." +
+            "\nRequires an app restart to take effect." +
+            "\n\nWARNING: this feature is EXPERIMENTAL and not all security precautions are implemented." +
+            " Do not enable on production accounts.",
+        defaultValue = { false },
+        isFinished = false,
+    ),
     Knock(
         key = "feature.knock",
         title = "Ask to join",

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/RustMatrixClientFactory.kt

@@ -127,6 +127,7 @@ class RustMatrixClientFactory @Inject constructor(
                     TrustRequirement.UNTRUSTED
                 }
             )
+            .enableShareHistoryOnInvite(featureFlagService.isFeatureEnabled(FeatureFlags.EnableKeyShareOnInvite))
             .run {
                 // Apply sliding sync version settings
                 when (slidingSyncType) {

libraries/matrix/impl/src/test/kotlin/io/element/android/libraries/matrix/impl/fixtures/fakes/FakeFfiClientBuilder.kt

@@ -41,6 +41,7 @@ class FakeFfiClientBuilder : ClientBuilder(NoPointer) {
     override fun slidingSyncVersionBuilder(versionBuilder: SlidingSyncVersionBuilder) = this
     override fun userAgent(userAgent: String) = this
     override fun username(username: String) = this
+    override fun enableShareHistoryOnInvite(enableShareHistoryOnInvite: Boolean): ClientBuilder = this
 
     override suspend fun buildWithQrCode(qrCodeData: QrCodeData, oidcConfiguration: OidcConfiguration, progressListener: QrLoginProgressListener): Client {
         return FakeFfiClient()