Introduce a RxObservable typealias to workaround SE-0395 conflict

In Swift 5.9 / Xcode 15 beta 2, Apple introduced a re-export of its own protocol
called Observable, under the Observation framework.

This protocol conflicts with RxSwift.Observable, which causes projects to fail compilation.

As a workaround, this PR introduces a new RxObservable typealias which can be used instead
of the Observable symbol where relevant (See #2532 for a quick find-and-replace to assist)

RxSwift/Observable.swift

@@ -9,6 +9,9 @@
 /// A type-erased `ObservableType`. 
 ///
 /// It represents a push style sequence.
+
+public typealias RxObservable<Element> = RxSwift.Observable<Element>
+
 public class Observable<Element> : ObservableType {
     init() {
 #if TRACE_RESOURCES