Expose a global search service from the client

Wrap the SDK's SearchService in a SearchServiceProxy that publishes typed
message results and paginates on demand, expose it on the ClientProxy, and
enable the search index store on the session. Also opens up the timeline item
factory's message content builder so the proxy can reuse it.

ElementX/Sources/Services/Client/ClientProxyProtocol.swift

@@ -155,6 +155,8 @@ protocol ClientProxyProtocol: AnyObject {
     
     var spaceService: SpaceServiceProxyProtocol { get }
     
+    var searchService: SearchServiceProxyProtocol { get }
+    
     var capabilities: HomeserverCapabilitiesProxyProtocol { get }
     
     var isReportRoomSupported: Bool { get async }