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?

Start by comparing the sagemaker.feature_store module in SDK 2.x with the SDK 3.x source and documentation. Determine whether Feature Store support is missing, relocated, deprecated, or redesigned. Done means either implementing the feature in SDK 3.x or documenting a supported alternative and migration path.

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