Add internal/database as a generic layer to access DB [Part 1]

What does this MR do and why?

Add internal/database as a generic layer to access DB

• This use MultiQuery approach to have a single query for multiple databases
• The DB layer implements minimal amount of functions universally supported by Spanner and PG
• Support Postgres and Spanner

References

• Slice of the: Draft: PoC: Transactional Behavior on Topology ... (!308)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.