Add covv_host as a new DB field

This PR adds a new TCVD location-treap query using a new field: covv_host. In this PR we will add to the TCVD database only (in PR 53 the same field is added as a static location). Next, we need to provide JSON snapshots containing the newly mentioned field.

As we can see in the PR content, we need only to add it to the fields mentioned in constants.hpp, and, then the new field becomes available in the query object.

The PR also presents an example of how to implement unit and integration tests for a new query.