Change @smithy/types to a peer dependency

Our library does not really depend on any specific version of
smithy/types, we have to use whatever the environment dictates (e.g.
mostly depending on which version of the AWS SDK is used inside the
consumers project).

As result we should not install @smithy/types on our own as it does not
make sense to ever install a separate version from the main version used
inside the project.

While we use types from the package in our function signatures
(`MetadataBearer`), our code actually does do not depend on a specific
shape of the type. As a result we are confident in our open-ended
version constraint.

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- `@smithy/types` is now a peer dependency. This should allow usage
+  with a broader range of `aws-sdk-client-mock` versions without feer to install
+  conflicting versions.
+
 ## [5.0.0] - 2024-12-15
 
 ### Added