manifest spec doesn't specify `project.config` key

See: https://github.com/apache/openwhisk-wskdeploy/pull/1078
Associated Doc: https://github.com/apache/openwhisk-wskdeploy/blob/master/docs/wskdeploy_apigateway_open_api_spec.md

Spec missing this key: https://github.com/apache/openwhisk-wskdeploy/tree/master/specification

Read docs/wskdeploy_apigateway_open_api_spec.md and inspect the specification directory, using pull request #1078 as context. Done means the manifest specification documents the project.config key consistently with the associated documentation.

Written by the indexing model from the issue text.