Add list_datasets_by_platform endpoint

[nathandf]

• Implement the project structure used by the Models API. Ex. presentation / application / domain / infra / bootstrap
• Rig up the existing HuggingFaceClient impls of ListDatasetsClient and GetDatasetClient to the new ClientProvider. The methods to provide the HuggingFace client should be analogous to the ListModelsClient and the GetModelClient.
• Add the utoipa annotations to each route handler in the presentation layer for auto-generation of the Datasets API's Open API Spec