Endpoint discovery for DynamoDB and others

Services such as DynamoDB and Timestream make use of a feature called endpoint discovery. Currently the SDK supports it only for Timestream, and the DynamoDB part is unimplemented. This issue tracks implementing it generically so that DynamoDB and all future services that use it in the future will work.

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Start by locating the existing Timestream endpoint-discovery support and tracing how the Rust SDK handles service endpoints. Define a generic approach that also covers DynamoDB and future endpoint-discovery services, then verify that the affected services use it correctly.

Written by the indexing model from the issue text.