[Multi-Tenant] `backend` tenants table

[njlie]

Multi-Tenancy is based on a mapping of tenant ids to api secrets and endpoints. It should also maintain rows for some basic out-of-band identifying information.

The schema for the tenants table is as follows:

tenants {
  id: uuid
  email: string
  publicName: string
  apiSecret: string
}

• Create migration for backend tenant table
• Create model.ts file for tenant table

[mkurapov]

This table should exist on a separate schema from the one that currently exists in the backend package.

Would it make sense to have this table under the public/default schema, and all of the tenanted-resource tables under a "tenant" schema?