Use untyped JSON for Gremlin responses

Today we use GraphSON v3 with types. This results in a very large response body, which is full of a lot of type information that Graph Explorer does not utilize much.

We should be able to switch to the untyped GraphSON format and everything should continue to work as it does today. This is evident in the openCypher support, since the openCypher result structure is nearly identical to the untyped GraphSON format.

This change will require a decent amount of testing to make sure the system continue to work with new and old versions of Gremlin and Neptune.

Start by locating the Gremlin response handling and the existing openCypher result handling, then review any response-compatibility tests. Change the response format only after confirming how typed and untyped GraphSON are consumed; done means Graph Explorer continues to work with new and old Gremlin and Neptune versions, with coverage for both formats.

Written by the indexing model from the issue text.