Allow SSO OAuth callback port override

[ashenm]

Fixes #9148

This change adds support allowing OAuth callback address port override used by commands like aws sso login via environment variable AWS_OAUTH_CALLBACK_SERVER_PORT

The main use case is containerized and remote development environments, where the default localhost callback port may not be suitable or easily reachable. By allowing the port to be configured externally, the login flow becomes more flexible across Docker, dev containers, remote workspaces, and other non-standard local setups.

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[ashovlin]

Hey @ashenm, I'd prefer we start with an option like --redirect-port in #9148.

We try to ensure that AWS_ environment variables work across SDKs and tools, so if going with AWS_OAUTH_CALLBACK_SERVER_PORT we'd want to coordinate with at least the AWS Tools for PowerShell team, and update more documentation. Starting with just an AWS CLI option is simpler, and part of the generated documentation.

Will this approach work for your use case?