Update ChainSetup and IdentityChain interface

[arandito]

Description

This PR refactors ChainSetup to store the resolved profile name instead of a parsed profile Section.

Downstream providers (e.g. assume-role) need ChainSetup to store the resolved profile name for cycle tracking. Storing the parsed Section alongside it would make the profile reachable two ways, setup.profile and setup.profile_file.get_profile(setup.profile_name), duplicating state. Using profile_name as the single source of truth lets providers read values consistently through MergedConfig.get(profile_name, key). That call also performs the string validation the previous _get_string helpers provided, so each provider no longer needs its own copy of that logic.

This PR also renames profile_file/profile_name_override to config_file/profile_name for consistency, and updates the EcsContainer module suggestion from aws-credentials-ecs to aws-credentials-http.

Testing

• Updated tests with new interfaces
• Ran make check-py and make test-py.

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