Provide lang-independent API to set per-project preferred connection

PHP editor already uses concept of per-project DB connection, LSP could use that as well for Java projects. The "per-project preferred DB connection" should probably move to the core DB module + SPI should be implemented by PHP + Java (maven, gradle ?) to persist the setting within project's metadata somehow.

An API should be made for clients to find a DB connection "suitable" for a file (from its project).