feat(model): add SlackIconObject composition object for CardBlock.slackIcon

The slackIcon field on CardBlock should be a composition object with
type "icon" and a name field, not a plain String.

https://docs.slack.dev/reference/block-kit/composition-objects/slack-icon-object

Author: srtaalej

slack-api-model/src/main/java/com/slack/api/model/block/CardBlock.java

@@ -1,5 +1,6 @@
 package com.slack.api.model.block;
 
+import com.slack.api.model.block.composition.SlackIconObject;
 import com.slack.api.model.block.composition.TextObject;
 import com.slack.api.model.block.element.BlockElement;
 import com.slack.api.model.block.element.ImageElement;
@@ -39,10 +40,10 @@ public class CardBlock implements LayoutBlock {
     private ImageElement icon;
 
     /**
-     * The name of a built-in Slack icon displayed next to the title and subtitle.
+     * A built-in Slack icon displayed next to the title and subtitle.
      * Mutually exclusive with {@code icon}.
      */
-    private String slackIcon;
+    private SlackIconObject slackIcon;
 
     /**
      * The title of the card. Maximum length for the text in this field is 150 characters.

slack-api-model/src/main/java/com/slack/api/model/block/composition/BlockCompositions.java

@@ -87,4 +87,14 @@ public static FeedbackButtonObject feedbackButton(ModelConfigurator<FeedbackButt
     public static SlackFileObject slackFile(ModelConfigurator<SlackFileObject.SlackFileObjectBuilder> configurator) {
         return configurator.configure(SlackFileObject.builder()).build();
     }
+
+    // SlackIconObject
+
+    public static SlackIconObject slackIcon(ModelConfigurator<SlackIconObject.SlackIconObjectBuilder> configurator) {
+        return configurator.configure(SlackIconObject.builder()).build();
+    }
+
+    public static SlackIconObject slackIcon(String name) {
+        return SlackIconObject.builder().name(name).build();
+    }
 }

slack-api-model/src/main/java/com/slack/api/model/block/composition/SlackIconObject.java

@@ -0,0 +1,25 @@
+package com.slack.api.model.block.composition;
+
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+/**
+ * Defines a built-in Slack icon that can be displayed next to the title and subtitle of a card block.
+ *
+ * @see <a href="https://docs.slack.dev/reference/block-kit/composition-objects/slack-icon-object">document</a>
+ */
+@Data
+@Builder
+@NoArgsConstructor
+@AllArgsConstructor
+public class SlackIconObject {
+    public static final String TYPE = "icon";
+    private final String type = TYPE;
+
+    /**
+     * The name of the built-in Slack icon.
+     */
+    private String name;
+}