Add aggregate configuration support and update documentation

- Introduced AggregateConfig to manage aggregation settings in the gateway configuration.
- Added validation for reserved alias 'agg' and included tests for aggregate alias handling.
- Updated config.example.yaml to demonstrate aggregate configuration options.
- Enhanced README.md to include information about the new aggregation endpoint and its usage.
- Modified gateway.go to integrate the new aggregate handler for processing aggregation requests.

internal/config/config_test.go

@@ -62,3 +62,39 @@ func TestValidateDuplicateAlias(t *testing.T) {
 		t.Fatal("expected error")
 	}
 }
+
+func TestValidateReservedAggAlias(t *testing.T) {
+	c := &Config{
+		Servers: []Server{
+			{Alias: "agg", BaseURL: "http://x:1"},
+		},
+	}
+	if err := c.Validate(); err == nil {
+		t.Fatal("expected error for reserved alias agg")
+	}
+}
+
+func TestValidateAggregateIncludeAliases(t *testing.T) {
+	c := &Config{
+		Servers: []Server{
+			{Alias: "a", BaseURL: "http://x:1"},
+		},
+		Aggregate: &AggregateConfig{
+			IncludeAliases: []string{"a"},
+		},
+	}
+	if err := c.Validate(); err != nil {
+		t.Fatal(err)
+	}
+	c2 := &Config{
+		Servers: []Server{
+			{Alias: "a", BaseURL: "http://x:1"},
+		},
+		Aggregate: &AggregateConfig{
+			IncludeAliases: []string{"nope"},
+		},
+	}
+	if err := c2.Validate(); err == nil {
+		t.Fatal("expected error for unknown include alias")
+	}
+}