feat(AnswersClient): V1 implementation

[PLNech]

Q	A
New feature?	yes/no
Need Doc update	yes

Describe your change

Adds a first implementation of the Answers API's prediction endpoint.

Design considerations

• As the API is still in beta, this implem doesn't strongly type Answers parameters like query, queryLanguages, or returnExtractAttribute until these have a finalized status and naming.
• I made the index a parameter, to allow for the same AnswersClient to target different indices successively - can be reconsidered if it makes it inconsistent with this client's users' expectations.