schema/field: add DefaultFunc for numeric types and add support for it in entc (#1153)

* entc/load: allow defaultfuncs on numeric types

* schema/field: add DefaultFunc on numeric types

* docs: document DefaultFunc better

* chore: update generated files

* pr: address issues

* docs: updates on faq, address requested changes

schema/field/internal/numeric.tmpl

@@ -108,6 +108,13 @@ func (b *{{ $builder }}) Default(i {{ $t }}) *{{ $builder }} {
 	return b
 }
 
+// DefaultFunc sets the function that is applied to set the default value
+// of the field on creation.
+func (b *{{ $builder }}) DefaultFunc(fn interface{}) *{{ $builder }} {
+	b.desc.Default = fn
+	return b
+}
+
 // Nillable indicates that this field is a nillable.
 // Unlike "Optional" only fields, "Nillable" fields are pointers in the generated field.
 func (b *{{ $builder }}) Nillable() *{{ $builder }} {
@@ -191,6 +198,9 @@ func (b *{{ $builder }}) Annotations(annotations ...schema.Annotation) *{{ $buil
 
 // Descriptor implements the ent.Field interface by returning its descriptor.
 func (b *{{ $builder }}) Descriptor() *Descriptor {
+	if b.desc.Default != nil {
+		b.desc.checkDefaultFunc({{ $t }}Type)
+	}
 	return b.desc
 }