[Proof-of-concept] New mechanism to define vendor specific queries in the portal

Brian, please review and consider this new pattern/mechanism of custom-sql loading.

The aim is allow developers to define custom vendor specific queries. This way we can tune any query using specific vendor capabilities (non-standard functions, constructions, idioms whatever), instead of trying to write standard/agnostic queries all the time (which are usually most inefficient and complex than specific ones)

The approach is quite straightforward: define a new naming pattern for custom-sql xml files: `custom-sql/model.xml` for common queries (standard ones) and `custom-sql/model-vendor.xml` for specific ones. The latter overwrites to former, so when you get one query, you'll have the specific one for current vendor (if exists) or the common one.

Looking forward your feedback!