Enable independent crate versioning for SDK crates (#1540)

* Update `hydrate-readmes` to take versions from `versions.toml`
* Add cleanup TODO comments to `changelogger`
* Update SDK readme template
* Split up `generate-version-manifest` subcommand
* Eliminate the `aws.sdk.version` property
* Fallback to model hash comparison if model metadata claims no changes
* Add `acquire-base-image` to `release.yml`
* Use empty model metadata for SDK generation in CI
* Fix the `aws-config` version number in SDK crate readmes