Avocado should flag a readme where the "latest" tag does not contain all APIs

The motivation for this request is readme files like:

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/marketplacecatalog/data-plane/readme.md

that have a "latest" tag (newer than the default tag, but not the default) that does not contain all the APIs defined in the default tag. In this case, the latest tag simply omits the main REST API definition, marketplacecatalog.json.

Avocado does check "MISSING_APIS_IN_DEFAULT_TAG", but this checks the default tag rather than latest tag and also is not enabled for data-plane.

Start by locating Avocado's MISSING_APIS_IN_DEFAULT_TAG check and the data-plane validation configuration. Compare its behavior with the marketplacecatalog readme and marketplacecatalog.json example linked in the issue, focusing on the latest tag. Done means the validation detects APIs absent from latest where appropriate and has coverage for data-plane readmes.

Written by the indexing model from the issue text.