Add client_credentials to the list of grant types supported by Reddit

src/OpenIddict.Client.WebIntegration/OpenIddictClientWebIntegrationHandlers.cs

@@ -545,6 +545,8 @@ public static partial class OpenIddictClientWebIntegrationHandlers
                 context.Request["access_type"] = options.AccessType;
             }
 
+            // By default, Reddit doesn't return a refresh token but
+            // allows sending a "duration" parameter to retrieve one.
             else if (context.Registration.ProviderName is Providers.Reddit)
             {
                 var options = context.Registration.GetRedditOptions();

src/OpenIddict.Client.WebIntegration/OpenIddictClientWebIntegrationProviders.xml

@@ -136,6 +136,7 @@
                      TokenEndpoint="https://www.reddit.com/api/v1/access_token"
                      UserinfoEndpoint="https://oauth.reddit.com/api/v1/me">
         <GrantType Value="authorization_code" />
+        <GrantType Value="client_credentials" />
         <GrantType Value="refresh_token" />
 
         <TokenEndpointAuthMethod Value="client_secret_basic" />

src/OpenIddict.Client.WebIntegration/OpenIddictClientWebIntegrationProviders.xsd

@@ -64,6 +64,7 @@
                                 <xs:simpleType>
                                   <xs:restriction base="xs:string">
                                     <xs:enumeration value="authorization_code" />
+                                    <xs:enumeration value="client_credentials" />
                                     <xs:enumeration value="implicit" />
                                     <xs:enumeration value="refresh_token" />
                                   </xs:restriction>