feat!: Add hooks to allow app modules to add things to state-sync (#10961)

## Description

Closes: #7340

- Support registering multiple snapshotters in snapshot manager.
- Append the extension snapshotters to existing snapshot stream.

~TODO: testing.~
- existing tests are fixed






---

### Author Checklist

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I have...

- [ ] 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
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
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- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] 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
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

baseapp/baseapp.go

@@ -260,6 +260,22 @@ func DefaultStoreLoader(ms sdk.CommitMultiStore) error {
 	return ms.LoadLatestVersion()
 }
 
+// CommitMultiStore returns the root multi-store.
+// App constructor can use this to access the `cms`.
+// UNSAFE: only safe to use during app initialization.
+func (app *BaseApp) CommitMultiStore() sdk.CommitMultiStore {
+	if app.sealed {
+		panic("cannot call CommitMultiStore() after baseapp is sealed")
+	}
+	return app.cms
+}
+
+// SnapshotManager returns the snapshot manager.
+// application use this to register extra extension snapshotters.
+func (app *BaseApp) SnapshotManager() *snapshots.Manager {
+	return app.snapshotManager
+}
+
 // LoadVersion loads the BaseApp application version. It will panic if called
 // more than once on a running baseapp.
 func (app *BaseApp) LoadVersion(version int64) error {

baseapp/options.go

@@ -210,7 +210,7 @@ func (app *BaseApp) SetSnapshotStore(snapshotStore *snapshots.Store) {
 		app.snapshotManager = nil
 		return
 	}
-	app.snapshotManager = snapshots.NewManager(snapshotStore, app.cms)
+	app.snapshotManager = snapshots.NewManager(snapshotStore, app.cms, nil)
 }
 
 // SetSnapshotInterval sets the snapshot interval.