Add option to override db url when instantiating client

[steebchen]

As opposed to setting/overriding it via an env var

something like

client := db.NewClient(db.WithURL("..."))

related: #166

Internal notes:

Set this QE flag

--overwrite-datasources <overwrite-datasources>
Base64 encoded datasource urls, overwriting the ones in the schema [env: OVERWRITE_DATASOURCES=]