Feat: Feature Store in Sagemaker SDK v3

**Description**
While reviewing the documentation and source code for the SageMaker SDK 3.x, I noticed that the sagemaker.feature_store module—available in SDK 2.x—appears to be missing. I could not find references to this feature in the current SDK or its documentation.

**Use Case**
I am in the process of migrating part of an existing SageMaker application from SDK 2.x to SDK 3.x. The application relies on sagemaker.feature_store for managing feature groups and interacting with the SageMaker Feature Store. The absence of this module is blocking the migration.

**Expected Behavior**
Feature Store functionality should be available in SDK 3.x, either under the same namespace or a clearly documented alternative. If the feature has been relocated, deprecated, or redesigned, clear migration guidance would be helpful.

**Additional Question**
Is Feature Store support planned for SDK 3.x? If so, is there an expected timeline or recommended migration path?