Start implementing FTUE splash screens.

Present splash screens from AuthenticationViewController

RiotSwiftUI/Modules/Onboarding/SplashScreen/View/OnboardingSplashScreenPageIndicator.swift

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+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import SwiftUI
+
+@available(iOS 14.0, *)
+struct OnboardingSplashScreenPageIndicator: View {
+    @Environment(\.theme) private var theme
+    
+    let pageCount: Int
+    let pageIndex: Int
+    
+    var body: some View {
+        HStack {
+            ForEach(0..<pageCount) { index in
+                Circle()
+                    .frame(width: 8, height: 8)
+                    .foregroundColor(index == pageIndex ? .accentColor : theme.colors.quarterlyContent)
+            }
+        }
+    }
+}