Typescript: DateTime properties are mapped to Date objects but they should be string

As the title says, the typescript interface generator maps the date/datetime/time types to Typescript's Date object (https://github.com/api-platform/client-generator/blob/master/src/generators/TypescriptInterfaceGenerator.js#L47).

However, API Platform by default serializes DateTime objects as ISO 8601 strings, so we should change the type to `string` I think, otherwise we will run into all sorts of typescript errors when trying to use the date string (which the TS compiler would see as a Date object).

I could do a PR with the change from Date to string, however I have another question: Can the user change how PHP DateTimeInterface is serialized by Api Platform (i.e. from string to timestamp)? If so, how could we tell this to the client generator?