refactor: replace middleware with explicit operation phases

[lucix-aws]

Middleware has failed the test of time:

• writing middleware is cumbersome given the transport-agnostic interface
• modifying the operation stack is fragile and can subtly introduce severe runtime breaks (see #aws-sdk-go-v2/2370)
• it's nigh impossible to figure out what an operation is meant to do by looking at its implementation, since that implementation is a giant list of middleware add operations. the order within is not clear
• due to the way middleware works by decorating functions, stack traces from within an operation call are basically unreadable

The intent is to replace middleware with an explicit operation workflow and instead expose customization through interceptors (#471).