Feature Request: From support for @mixin structs

[spf5000]

My team currently leverages the @mixin feature in smithy to ensure types are shares across APIs and flatten API outputs instead of returning nested structs. This helps the APIs look "cleaner", but is actually a bit painful to work with in Rust because we have to create a type conversion for each type that references the mixin.

This may not be possible because the @mixin gets expanded before smithy-rs executes, but it'd be really nice to have some form of auto-generated From for Mixin support. That would let us internally work off the data in the mixin in our functions and skip the boiler plate conversions all of the time.