feat: ADR-040: Implement KV Store with decoupled storage and SMT (#9892)

## Description

Resolves: https://github.com/cosmos/cosmos-sdk/issues/10117

Implements a `CommitKVStore` which separates the concerns of state storage and state commitment according to [ADR-040](https://github.com/cosmos/cosmos-sdk/blob/eb7d939f86c6cd7b4218492364cdda3f649f06b5/docs/architecture/adr-040-storage-and-smt-state-commitments.md).

---

### Author Checklist

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- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
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- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules) - n/a
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
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- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
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- [ ] manually tested (if applicable)

docs/core/store.md

@@ -222,6 +222,26 @@ When `Store.{Get, Set}()` is called, the store forwards the call to its parent,
 
 When `Store.Iterator()` is called, it does not simply prefix the `Store.prefix`, since it does not work as intended. In that case, some of the elements are traversed even they are not starting with the prefix.
 
+## New Store package (`store/v2`)
+
+The SDK is in the process of transitioning to use the types listed here as the default interface for state storage. At the time of writing, these cannot be used within an application and are not directly compatible with the `CommitMultiStore` and related types.
+
+### `BasicKVStore` interface
+
+An interface providing only the basic CRUD functionality (`Get`, `Set`, `Has`, and `Delete` methods), without iteration or caching. This is used to partially expose components of a larger store, such as a `flat.Store`.
+
+### Flat Store
+
+`flat.Store` is the new default persistent store, which internally decouples the concerns of state storage and commitment scheme. Values are stored directly in the backing key-value database (the "storage" bucket), while the value's hash is mapped in a separate store which is able to generate a cryptographic commitment (the "state commitment" bucket, implmented with `smt.Store`).
+
+This can optionally be constructed to use different backend databases for each bucket.
+
+<!-- TODO: add link +++ https://github.com/cosmos/cosmos-sdk/blob/v0.44.0/store/v2/flat/store.go -->
+
+### SMT Store
+
+A `BasicKVStore` which is used to partially expose functions of an underlying store (for instance, to allow access to the commitment store in `flat.Store`).
+
 ## Next {hide}
 
 Learn about [encoding](./encoding.md) {hide}