feat: Allow build objs to have default values

packages/ra-cubicweb/src/Schema.ts

-export type BuildObj = {
-  type: "String" | "Date" | "Float" | "Int" | "Boolean";
+type BuildObjBase<T extends string> = {
+  type: T;
   required?: boolean;
 };
 
@@ -3,6 +3,33 @@
   required?: boolean;
 };
 
+interface StringBuildObj extends BuildObjBase<"String"> {
+  default?: string;
+}
+
+interface DateBuidObj extends BuildObjBase<"Date"> {
+  default?: Date | "NOW" | "TODAY";
+}
+
+interface FloatBuildObj extends BuildObjBase<"Float"> {
+  default?: number;
+}
+
+interface IntBuildObj extends BuildObjBase<"Int"> {
+  default?: number;
+}
+
+interface BooleanBuildObj extends BuildObjBase<"Boolean"> {
+  default?: boolean;
+}
+
+export type BuildObj =
+  | StringBuildObj
+  | DateBuidObj
+  | FloatBuildObj
+  | IntBuildObj
+  | BooleanBuildObj;
+
 export type Cardinality = "*" | "1" | "?" | "+";
 
 export type CardinalityPair = `${Cardinality}${Cardinality}`;