Customization Abstractions & Config Generation (#96)

* Customization Abstractions & Config Generation

A previous PR supported idempotency tokens with a hardcoded config implementation. This commit replaces it with the "real thing,"
supported by the `NamedSectionGenerator` abstraction. The intention idea is that sections can be typesafe, well documented, and evolve as compile
errors instead of silent failures.

Sections can also bring along state / generation context to allow for situation-specific customization. This API is still very experimental
and is expected to evolve as we continue to generate a fully customized DynamoDB client.

HttpProtocolGeneratorTest & integration tests provide complete test coverage—a unit test was also added to allow quicker iterations when adding customizations.

* Use typealias

* Fix missed merge conflicts, rename to member