Remove CurrentValueSubject as protocol requirement (#807)

* Add CurrentValuePublisher

* Refactor to use CurrentValuePublisher

* Add comment

ElementX/Sources/Other/CurrentValuePublisher.swift

@@ -0,0 +1,45 @@
+//
+// Copyright 2023 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 Combine
+
+/// A wrapper of CurrentValueSubject.
+/// The purpose of this type is to remove the possibility to send new values on the underlying subject.
+struct CurrentValuePublisher<Output, Failure: Error>: Publisher {
+    private let subject: CurrentValueSubject<Output, Failure>
+    
+    init(_ subject: CurrentValueSubject<Output, Failure>) {
+        self.subject = subject
+    }
+    
+    init(_ value: Output) {
+        self.init(CurrentValueSubject(value))
+    }
+    
+    func receive<S>(subscriber: S) where S: Subscriber, Failure == S.Failure, Output == S.Input {
+        subject.receive(subscriber: subscriber)
+    }
+    
+    var value: Output {
+        subject.value
+    }
+}
+
+extension CurrentValueSubject {
+    func asCurrentValuePublisher() -> CurrentValuePublisher<Output, Failure> {
+        .init(self)
+    }
+}