Move `alb_health_check` module out of the `plugin` module (#2865)

The current module location is misleading, since you never want to run
the layer as a plugin: plugins always run after routing, and the health
check should be enacted before routing. Examples have been corrected,
and a test has been added.

## 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
- [ ] 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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