[google|sql] Initial support for Google Cloud SQL

This commit adds the initial support for Google Cloud SQL:
- Adds a new service 'SQL' to the existing 'Google' provider
- Creates new shared methods to be reused by different services
- Add requests, models and tests for Tiers

See #2976 for more details
I will do, but I prefer to merge this as standalone (it's cleaner and it doesn't break the actual compute calls), and later refactor compute.