store registry tokens on S3

Right now registry tokens are stored within postgres, which means that in case of a postgres database downtime (or maintenance), the usage reporting feature might be impacted as a token might not be stored in memory of tokens/usage reporting service.

By moving the storage of the tokens to S3, we avoid this bottleneck as scaling/upgrading postgres usually implies a downtime.

As part of implementing this task we need to have a plan for migrating the existing tokens from pg to s3.

Also related to https://github.com/kamilkisiela/graphql-hive/issues/124