feat(api): add possibility to retrieve user schemas (#7614)

This PR extends the user schema service (V3 API) with the possibility to ListUserSchemas and GetUserSchemaByID.
The previously started guide is extended to demonstrate how to retrieve the schema(s) and notes the generated revision property.

internal/eventstore/handler/v2/statement.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/zitadel/logging"
+	"golang.org/x/exp/constraints"
 
 	"github.com/zitadel/zitadel/internal/database"
 	"github.com/zitadel/zitadel/internal/eventstore"
@@ -504,6 +505,16 @@ func NewJSONCol(name string, value interface{}) Column {
 	return NewCol(name, marshalled)
 }
 
+func NewIncrementCol[Int constraints.Integer](column string, value Int) Column {
+	return Column{
+		Name:  column,
+		Value: value,
+		ParameterOpt: func(placeholder string) string {
+			return column + " + " + placeholder
+		},
+	}
+}
+
 type Condition func(param string) (string, []any)
 
 type NamespacedCondition func(namespace string) Condition