Add schema for report JSON and improve reporting

[ivbeg]

Right now JSON file of the metadata scanning report is not structured well enough. Improvements should include:

• [ ] Add Cerberus schema (more info https://docs.python-cerberus.org)
• [ ] Add scanning datetime
• [ ] Add source info: source type, filename, connection string e.t.c. Make sure no secrets in connection string
• [ ] Move 'table' to 'source' subtag
• [ ] Add tests to validate reports with Cerberus validator