AWS AppConfig (appconfig) + AppConfig Data (appconfigdata) implementation
for FakeCloud.
One crate serves both AWS model-services. They share the SigV4 signing name
appconfig; the single service handler splits control-plane traffic
(/applications/..., /deploymentstrategies/..., /extensions/...) from
data-plane traffic (/configurationsessions, /configuration) on the URL
path, mirroring the OpenSearch (es + opensearch) dual-model crate.