feat: SQL API req.meta expose target dbname

**Is your feature request related to a problem? Please describe.**
We're currently implementing a multi-tenancy authorization scenario and would like to match the target database for a connection through SQL API to data contained in the JWT of the user. We're able to extract the claims from the token in our cube.js but the target dbname isn't available in the context.

**Describe the solution you'd like**
In addition to the existing metadata we propose to add also the dbname. This field is already stored in the session, but currently not exposed to req.meta

**Describe alternatives you've considered**
With the available metadata we were thinking about re-using the changeUser or applicationName. Nevertheless we think it would be beneficial to expose the intended target database on a separate optional field.

**Additional context**
I'll link a PR with a proposal for an implementation.