change BidResponse.ext to be a JsonElement

[renam]

No description provided.

[standa-dev]

might also be good to test when the use_new_renderer is missing or false

[JasonSznol]

I think the suggestion @marcsantiago made about strongly typing the ext field is a safer approach and does not have the trade off of coupling the deserialization with Json; for comparison both iOS and the Go module in this repo are using strongly typed ext objects.

In some future if we wanted to make this more generic we can leverage contextual serialization and define the ext field serializer within the request module of the SDK to avoid having to make updates in this module should the ext field change again.