add POST / endpoint for default decisions

[charlesdaniels]

As described here, the OPA API allows for "default decisions" to be made against a configurable "default rule". To support enabling this feature from the SDKs, I have added an appropriate endpoint to OpenAPI spec. Note that unlike the /v1/data/{path} endpoints, / does not require wrapping the input with {"input": ... }, nor does it wrap the result in {"result": ...}.