Decouple relation_embedder config from typesafe config

[kenoir]

What does this change?

This change implements a suggestion in a previous PR to have one place where we reference string based paths for typesafe config.

The suggestion is that services have classes that extend LambdaConfigurable and implement a build function that accepts typesafe config and provides a case class that extends ApplicationConfig.

How to test

• Run the tests, do they pass?

How can we measure success?

Simpler and clearer implementation of new lambda services that consume typesafe config.

Have we considered potential risks?

This change should functionally be a no-op.