Add new fetchMethod to specify legacy gateways

[bdrhn9]

Current implementation has two fetchMethod, api and gateway. When gateway is selected, the requester function tries to parse response with legacy schema as in here. If it fails, then it tries with the current schema. Adding an extra fetchMethod like gateway-old, we can distinctly select the schema going to be utilized for parsing.

[aquarat]

As noted in the meeting, this is low priority as the only side effect of the current situation is reduced compute efficiency.