Add tokenExchange grant type to list of allowed grant types for oauth2 shape

[Current spec](https://spec.openapis.org/oas/latest.html#fixed-fields-23) definition allows `implicit`, `password`, `clientCredentials`, and `authorizationCode` for oauth2 flows. This request is to include [tokenExchange](https://www.rfc-editor.org/rfc/rfc8693) in the list of grant types/flows.