Add HealthChat API and extended profile services (#9)

Expose OpenID tokens and homeserver credentials from the client proxy, and
add fork-only services for the HealthChat member directory backend
(api.healthchat.ch) plus a service that reads MSC4133 extended profile
fields directly from the homeserver. Groundwork for the members tab and
the profile screens.

Claude-Session: https://claude.ai/code/session_013qdTbAWsZj2quQzEkagXaS

ElementX/Sources/Services/Client/HomeserverCredentials.swift

@@ -0,0 +1,21 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Foundation
+
+/// The session's REST credentials for direct client-server API calls that the SDK doesn't cover.
+/// The access token can rotate after a refresh, so re-read these for every request and never persist them.
+nonisolated struct HomeserverCredentials: Equatable {
+    let homeserverURL: URL
+    let accessToken: String
+}
+
+extension HomeserverCredentials: CustomStringConvertible {
+    var description: String {
+        "HomeserverCredentials(homeserverURL: \(homeserverURL), accessToken: <redacted>)"
+    }
+}