Replace `DynConnector` and `HttpConnector` with `HttpClient` (#3011)

This PR removes the last usages of `DynConnector` and `HttpConnector`
from the `aws_smithy_client` crate with their counterparts in
aws-smithy-runtime-api and aws-smithy-runtime. It also introduces
`HttpClient` to make HTTP connector selection a smoother process, and
adds a runtime plugin that configures a default `HttpClient` so that
`Config` doesn't have to do that.

The `DnsService` from aws-config is also moved into aws-smithy-runtime
and refactored to be consistent with the other configurable traits in
the orchestrator since it will likely be used in the future for more
than just the ECS credentials provider.

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