feat: allow clear select value (#735)

src/fields/config/schema.ts

@@ -144,6 +144,7 @@ export const select = baseField.keys({
     joi.array().items(joi.string().allow('')),
     joi.func(),
   ),
+  isClearable: joi.boolean().default(false),
 });
 
 export const radio = baseField.keys({

src/fields/config/types.ts

@@ -214,6 +214,7 @@ export type SelectField = FieldBase & {
   type: 'select'
   options: Option[]
   hasMany?: boolean
+  isClearable?: boolean;
 }
 
 export type RelationshipField = FieldBase & {