Define GET /exchange-rates

Overview:
In order to limit the number of calls out to the public exchange rate API, e.g. by multiple instances of the invoice module, or by other modules which need exchange rate information, we should implement an exchange-rates API. This API will provide a central location for modules to obtains exchange-rate information and provides a caching layer.

where from and to are currency codes, e.g. USD, GBP

Approach:
Define a new interface "exchange-rates" w/ appropriately named permissions
Define a new exchange-rate schema:

{
  "to": "USD",
  "from": "EUR",
  "rate": 0.8929368694,
  "date": "2019-05-06"
}

See MODINVOICE-44 for the implementation details of this API.

Acceptance Criteria:

• RAML and ModuleDescriptor are updated
• Schemas and Schema Examples exist