Add Fitbit to the list of supported providers

src/OpenIddict.Client.WebIntegration/OpenIddictClientWebIntegrationHandlers.Userinfo.cs

@@ -212,6 +212,11 @@ public static partial class OpenIddictClientWebIntegrationHandlers
 
                 context.Response = context.Registration.ProviderName switch
                 {
+                    // Fitbit returns a nested "user" object.
+                    Providers.Fitbit => (JsonElement) context.Response["user"]
+                        is { ValueKind: JsonValueKind.Object } element ?
+                        new(element) : throw new InvalidOperationException(SR.FormatID0334("user")),
+
                     // StackExchange returns an "items" array containing a single element.
                     Providers.StackExchange => (JsonElement) context.Response["items"]
                         is { ValueKind: JsonValueKind.Array } element && element.GetArrayLength() is 1 ?

src/OpenIddict.Client.WebIntegration/OpenIddictClientWebIntegrationProviders.xml

@@ -178,6 +178,33 @@
     </Environment>
   </Provider>
 
+  <!--
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+  -->
+
+  <Provider Name="Fitbit" Documentation="https://dev.fitbit.com/build/reference/web-api/developer-guide/authorization/">
+    <Environment Issuer="https://www.fitbit.com/">
+      <Configuration AuthorizationEndpoint="https://www.fitbit.com/oauth2/authorize"
+                     TokenEndpoint="https://api.fitbit.com/oauth2/token"
+                     UserinfoEndpoint="https://api.fitbit.com/1/user/-/profile.json">
+        <CodeChallengeMethod Value="plain" />
+        <CodeChallengeMethod Value="S256" />
+
+        <TokenEndpointAuthMethod Value="client_secret_basic" />
+      </Configuration>
+
+      <!--
+        Note: Fitbit requires sending the "profile" scope to be able to use the userinfo endpoint.
+      -->
+
+      <Scope Name="profile" Default="true" Required="true" />
+    </Environment>
+  </Provider>
+
   <!--
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