updated spec to include a separate single-evaluation API description, and other cleanup

[ogazitt]

This PR cleans up a few discrepancies / inconsistencies between the interop scenario and the existing draft:

1. consistent treatment of case. all JSON keys are consistently lowercased, to match the examples, and match the interop payloads.
2. renamed the sections called "Access Evaluation API Request/Response" to "Access Evaluations Request/Response". these sections describe the boxcarred API.
3. added corresponding sections called "Access Evaluation API Request/Response" which describes the NON boxcarred version that is used in the interop scenario. All payloads are now consistent with the payloads spec.
4. cleaned up JSON formatting - all indentation now uses 2 spaces, as is standard. Most of the spec was consistent with this, but there were some places where 4 spaces were used, and some where 2 and 4 were mixed (!)
5. clarified the URLs for the requests to be /access/v1/evaluation and /access/v1/evaluations. Previously, in one place it was noted as /evaluations, and in another /access/v1/evaluations - now they are all consistent with the naming scheme we agreed on during our last call.

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