[FEATURE] Check response body payload

[dblock]

Is your feature request related to a problem?

The response is checked against spec, but not its payload content. Adding payload into a spec test doesn't do anything. We have a few instances of those such as in

opensearch-api-specification/tests/text_embedding_processor.yaml

Line 28 in 362c90c

acknowledged: true

where we expect a certain response.

What solution would you like?

1. Allow one to specify the expected partial or complete response.
2. Fail if the test spec has extra fields (right now payload is unused but it doesn't hurt to specify it, it should).