fix: allow multiple URI schemes during discovery

This commit addresses a potential problem which could arise
if the scheme of a URI obtained via CoRE Resource Discovery
was not the same as the URI scheme of the original discovery URI.
This was caused by the fact that only one ProtocolClient for
the original URI scheme was initialized.

This commit addresses the issue by introducing a cache for clients
which is expanded if more clients for different URI schemes are needed
during discovery. In the stop method, all clients are cleaned up
simultaneously.

lib/src/core/thing_discovery.dart

@@ -31,13 +31,14 @@ class DiscoveryException implements Exception {
 class ThingDiscovery extends Stream<ThingDescription>
     implements scripting_api.ThingDiscovery {
   /// Creates a new [ThingDiscovery] object with a given [thingFilter].
-  ThingDiscovery(this.thingFilter, this._servient)
-      : _client = _servient.clientFor(thingFilter.url.scheme) {
+  ThingDiscovery(this.thingFilter, this._servient) {
     _stream = _start();
   }
 
   final Servient _servient;
 
+  final Map<String, ProtocolClient> _clients = {};
+
   bool _active = true;
 
   @override
@@ -48,8 +49,6 @@ class ThingDiscovery extends Stream<ThingDescription>
 
   late final Stream<ThingDescription> _stream;
 
-  final ProtocolClient _client;
-
   Stream<ThingDescription> _start() async* {
     final discoveryMethod = thingFilter.method;
 
@@ -68,9 +67,23 @@ class ThingDiscovery extends Stream<ThingDescription>
     }
   }
 
+  ProtocolClient _clientForUriScheme(Uri uri) {
+    final uriScheme = uri.scheme;
+    var client = _clients[uriScheme];
+
+    if (client == null) {
+      client = _servient.clientFor(uriScheme);
+      _clients[uriScheme] = client;
+    }
+
+    return client;
+  }
+
   @override
   Future<void> stop() async {
-    await _client.stop();
+    final stopFutures = _clients.values.map((client) => client.stop());
+    await Future.wait(stopFutures);
+    _clients.clear();
     _active = false;
   }
 
@@ -89,7 +102,9 @@ class ThingDiscovery extends Stream<ThingDescription>
   }
 
   Stream<ThingDescription> _discoverDirectly(Uri uri) async* {
-    yield* _client
+    final client = _clientForUriScheme(uri);
+
+    yield* client
         .discoverDirectly(uri, disableMulticast: true)
         .asyncMap((content) => _decodeThingDescription(content, uri));
   }
@@ -149,8 +164,10 @@ class ThingDiscovery extends Stream<ThingDescription>
   ) async* {
     final Set<Uri> discoveredUris = {};
     final discoveryUri = uri.toLinkFormatDiscoveryUri(resourceType);
+    final client = _clientForUriScheme(uri);
+
     await for (final coreWebLink
-        in _client.discoverWithCoreLinkFormat(discoveryUri)) {
+        in client.discoverWithCoreLinkFormat(discoveryUri)) {
       final Iterable<Uri> parsedUris;
 
       try {