fix(sequelize): extend Inclusion interface with "required" property

This resolves errors such as the following when trying to use the
new property with a repository call:
```ts
Type '{ relation: string; scope: { where: { id: number; }; }; required: true; }'
is not assignable to type 'InclusionFilter'.
  Object literal may only specify known properties, and 'required'
does not exist in type 'Inclusion'.

53           required: true
```

Signed-off-by: KalleV <kvirtaneva@gmail.com>

Author: KalleV

extensions/sequelize/src/types.ts

@@ -3,6 +3,18 @@
 // This file is licensed under the MIT License.
 // License text available at https://opensource.org/licenses/MIT
 
+declare module '@loopback/repository' {
+  interface Inclusion {
+    /**
+     * Setting this option to true will result in an inner join query that
+     * explicitly requires the specified condition for the child model.
+     *
+     * @see https://loopback.io/pages/en/lb4/readmes/loopback-next/extensions/sequelize/#inner-join
+     */
+    required?: boolean;
+  }
+}
+
 /**
  * Interface defining the component's options object
  */