[App Configuration] Add Key-value snapshots feature

Key-value snapshots allow developers to create a point-in-time snapshot of their configuration store. These snapshots are immutable. They can be deleted, but will have soft-delete enforced. During creation, developers can add filters to their snapshots to filter what key-values get put inside. This allows developers to create immutable, composed views of their configuration store that they can release with their application.

Snapshots can be referenced inside of an application by snapshot name, offloading complex client configuration queries in application configuration. Due to their immutable nature, developers can confidently rollback to a last-known-good configuration in the event of an issue with newer configuration.

This feature will be available in `2022-11-01-preview` API version.

[Microsoft Internal] Here is the PR for this feature - https://github.com/Azure/azure-rest-api-specs-pr/pull/8095