[request]: add a `.use_local_endpoint(port)` method to `dynamodb::Config`

### Tell us about your request

add a `.use_local_endpoint(port)` method to `dynamodb::Config`

### Tell us about the problem you're trying to solve.

I want to use DynamoDbLocal

### Are you currently working around this issue?

https://docs.aws.amazon.com/sdk-for-rust/latest/dg/dynamodb-local.html but it's a lot of ceremony for a common use case

### Additional context

filed on behalf of @carllerche

### Community Note
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Start with dynamodb::Config and the DynamoDB Local guide linked in the issue to understand the current setup ceremony and endpoint configuration. Define the expected behavior of a `.use_local_endpoint(port)` convenience method and verify that it produces a usable DynamoDB Local configuration without requiring the documented manual steps.

Written by the indexing model from the issue text.