[feat] add reach-like functionality to object

In elastic#36804 we need to validate a `Partial<HttpConfig>` object which may
or may not have all the required keys attached to it.  This is the type
of use-case for `Joi.reach`, so we should expose a reach-like method on
the object validation class so that you can return the validator for a
specific key on an object.

packages/kbn-config-schema/src/types/object_type.test.ts

@@ -199,3 +199,19 @@ test('includes namespace in failure when wrong value type', () => {
 
   expect(() => type.validate(value, {}, 'foo-namespace')).toThrowErrorMatchingSnapshot();
 });
+
+test('reach returns a valid schema component', () => {
+  const type = schema.object({
+    foo: schema.boolean(),
+  });
+
+  const fooSchema = type.reach('foo');
+  const value = false;
+  expect(() => fooSchema.validate(value)).toBeTruthy();
+
+  try {
+    type.reach('bar');
+  } catch (err) {
+    expect(err.message).toBe('bar is not a valid part of this schema');
+  }
+});

packages/kbn-config-schema/src/types/object_type.ts

@@ -31,6 +31,8 @@ export type TypeOf<RT extends Type<any>> = RT['type'];
 export type ObjectResultType<P extends Props> = Readonly<{ [K in keyof P]: TypeOf<P[K]> }>;
 
 export class ObjectType<P extends Props = any> extends Type<ObjectResultType<P>> {
+  private props: Record<string, AnySchema>;
+
   constructor(props: P, options: TypeOptions<{ [K in keyof P]: TypeOf<P[K]> }> = {}) {
     const schemaKeys = {} as Record<string, AnySchema>;
     for (const [key, value] of Object.entries(props)) {
@@ -44,6 +46,7 @@ export class ObjectType<P extends Props = any> extends Type<ObjectResultType<P>>
       .default();
 
     super(schema, options);
+    this.props = schemaKeys;
   }
 
   protected handleError(type: string, { reason, value }: Record<string, any>) {
@@ -57,4 +60,9 @@ export class ObjectType<P extends Props = any> extends Type<ObjectResultType<P>>
         return reason[0];
     }
   }
+
+  reach(key: string) {
+    if (!this.props[key]) throw new Error(`${key} is not a valid part of this schema`);
+    return this.props[key];
+  }
 }