This is the implementation of the new DB connections API (grant proposal 2019).
Summary
The new API makes it available to QGIS core a new interface for provider connections and will allow to:
replace the provider specific QgsSettings management in QGIS4 (save/load connections from the settings) NOT IN SCOPE FOR NOW.
provide a unified API for common operations on DB connections:
executeSql and get the results
list tables names and properties and schemas
create a new vector table (no rasters for now)
create/rename/drop schemas and tables
vacuum
....
