Feature-gate http versions in aws-smithy-runtime-api (#3236)

## Motivation and Context
Without this, we will have http = 0.2 permanently in-tree.

## Description
- Add feature gate for http 02x. 
- ~Add http 1x as an experiment.~

## Testing
CI

## 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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