fix(types): add client config interface test with s3 example

[kuhe]

Issue

#4122

Our client config types are a mash of many different types due to being piped through many config resolvers.
The endpoints 2.0 changes modified the runtime type of the config and broke the type contract for resolvedEndpointConfig, which had assumed endpoint would be set.

This test package aims to stabilize the client config resolved interface.

Description

• this adds a private package that is only used for unit testing
• the tests check that a minimal and maximal configuration for a client (S3 used as example) resolve the expected fields

Testing

• added unit tests
• added build test for client api (S3 used as example)

[github-actions]

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