Add a "normalize" function to the config definition and remove that responsibility from the "apply" function

[jeskew]

This PR separates the current responsibilities of the apply function in a config property definition into two separate steps: a normalize function that converts user input to the subtype expected by the SDK and an apply function that modifies the middleware stack.

This allows two optimizations:

1. We can forgo calling the normalizer if the user did not supply anything for a property.
2. The apply function can be deferred until the config is fully normalized and hydrated with defaults. This smaller scope for apply is more or less equivalent to what a middleware customization would do, so we can look at removing apply once that customization type is available.

[chrisradek]

Closed as part of #142

[lock]

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