API allows arbitrary metric file requests

[macfarlandian]

What needs to be done? Why does it need to be done?
In addition to sending page-level batches, the spotlight-api backend application should allow clients to request one or more arbitrary files by name. Create a new endpoint that supports this and the corresponding client code in spotlight-client for consuming it.

Additional context
Metrics service design doc, which this will be a partial implementation of

Partial client prototype in #257