Add support for BehaviorMajorVersions (#3151)

## Motivation and Context
See [rendered
RFC](https://github.com/awslabs/smithy-rs/blob/df518bfb59c7498ab533fac4256ec0b977cee42c/design/src/rfcs/rfc0039_behavior_major_versions.md)

## Description
This add `BehaviorMajorVersions` to the SDK and wires them in up and
down the stack.

## Testing
- [x] lots of ITs / UTs

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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