[Multi-Tenant] Tenanted Rates endpoints

[njlie]

As quotes and outgoing payments are now tenanted, tenants should now be responsible for setting their own rates and hosting them at their own endpoint.

In the backend business logic, the existing rates service gets rates from an environment variable for a URL. The Rates service should instead be updated to require a tenantId when acquiring rates, so that it pulls from the tenant's ratesEndpoint when calculating rates.

• Update the RatesService.rates() method to retrieve rates based on the url from a provided tenant id.
• Update the RatesService to cache rates on a per-tenant basis.

Prerequisites

• [Multi-Tenant] backend tenants table #3103

[mkurapov]

Depends on #3127