Refine typings for `AutocompleteRequest.types` to allow for all variations allowed by the API

### Issue description

[In the documentation for the `placeAutocomplete` method](https://developers.google.com/maps/documentation/places/web-service/autocomplete#place_types) it is said that it supports

> Up to five values from [Table 1](https://developers.google.com/maps/documentation/places/web-service/supported_types#table1) or [Table 2](https://developers.google.com/maps/documentation/places/web-service/supported_types#table2)

separated by a vertical bar `|` as a valid value for the `types` parameter of the `autocomplete` endpoint.

However, in [`places/autocomplete.ts`](https://github.com/googlemaps/google-maps-services-js/blob/master/src/places/autocomplete.ts) the type of the `types` property is an enum [`PlaceAutocompleteType`](https://github.com/googlemaps/google-maps-services-js/blob/master/src/places/autocomplete.ts#L31), which supports a very limited set of types/filters for this API, which makes it impossible to use the method with the values from the aforementioned tables without tricking TypeScript with `as unknown as PlaceAutocompleteType` cast.

This contradicts the documentation of the endpoint, makes it impossible to use singular or combined values from the aforementioned tables and limits the capabilities of the autocompletion.

### Suggestion

If these capabilities of filtering by types are not to be deprecated by Google Maps API itself, I would suggest changing the type of `types` field to `string` to reach parity in filtering autocompletion predictions between raw API calls and this library.

I would also appreciate if anybody could suggest me another way to, for example, send `{ types: 'postal_code' }`, if there is one in this library and I'm not aware of it. It is supported in raw API calls and I see no reason why it should not be supported in the library.