[CD] [69842562] Support a configuration service

Support an optional configuration service. If enabled, it will be
exposed via the `/config` route. The README includes a section on the
expected API of a class that must be provided to use a configuration
service.
[CD] [69842562] Expect "callable" instead of class

In order to be more flexible with our API, expect the configuration
service adapter to respond to `call` instead of `new` and `get_config`.
This allows us to provide anything that is "callable", e.g., a block,
proc, lambda, a class with a `call` method, etc.

Also, the `use_config_service` boolean isn't required; remove it.
Merge pull request #10 from primedia/69842562-add-optional-configuration-service-support

[CD] [69842562] Support a configuration service