Allow MCPConfigRepo to output extra for MavenTask

Usage: `--artifact net.minecraft:client-extra`

Author: Jonathing

src/main/java/net/minecraftforge/mcmaven/impl/repo/mcpconfig/MCPConfigRepo.java

@@ -11,6 +11,7 @@
 import net.minecraftforge.mcmaven.impl.cache.Cache;
 import net.minecraftforge.mcmaven.impl.mappings.Mappings;
 import net.minecraftforge.mcmaven.impl.util.Artifact;
+import net.minecraftforge.mcmaven.impl.util.Constants;
 import net.minecraftforge.mcmaven.impl.util.POMBuilder;
 import net.minecraftforge.mcmaven.impl.util.Task;
 import net.minecraftforge.mcmaven.impl.util.Util;
@@ -98,6 +99,9 @@ public List<PendingArtifact> process(Artifact artifact, Mappings mappings) {
         if (isMappings)
             side = "joined";
 
+        if (side.endsWith("-extra"))
+            return processExtra(Constants.MC_GROUP + ':' + side.substring(0, side.length() - "-extra".length()), version);
+
         var mcp = this.get(Artifact.from("de.oceanlabs.mcp", "mcp_config", version, null, "zip"));
         var mcpSide = mcp.getSide(side);