[FT-2788]fix opa documents undefined return

[windmgc]

Summary

• Fix the example rego file in the OPA plugin documentation. Currently, the allowBoolean document does not have a default value, which means that if not evaluated the returned value will be undefined and the OPA decision response will be a {}, which will eventually cause a 500 error on Kong side. Can be reproduced by following the current version of document, https://docs.konghq.com/hub/kong-inc/opa/#usage. When trying request on the first time, the 500 error will occur.
• Add two missing config params, include_body_in_opa_input and include_parsed_json_body_in_opa_input

Reason

Fix https://konghq.atlassian.net/browse/FT-2788

Testing

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