Recognize dictionary type in codegen (#37206)

Summary:
Previously we allow `{[key:string]:Something}` in codegen, `Something` is type-checked but thrown away, generating a `GenericObjectTypeAnnotation`.
In this change, `Something` is added to `GenericObjectTypeAnnotation` as an optional field.
For downstream code such as C++ codegen, this change is **backward compatible**. It allows C++ codegen to produce a more precious type optionally.

## Changelog:

[General] [Added] - Recognize dictionary type in codegen

Pull Request resolved: https://github.com/facebook/react-native/pull/37206

Test Plan:
```
yarn jest react-native-codegen
yarn jest react-native-codegen-typescript-test
```

Reviewed By: cipolleschi

Differential Revision: D45563340

Pulled By: dmytrorykun

fbshipit-source-id: 9a9ce36df6ded6d42d35c3dcb6fb0eaca16c4458