feat(dot/sync): Gap sync state

### Description

The `GapSyncState` can be place in the file `dot/sync/gap_sync_state.go`, this file will be similar to `dot/state/grandpa.go`, example:

```go
type GapSyncInfo struct {
latestImportedBlock common.Hash
target common.Hash
}

type GapSyncState struct {
gapSyncDB database.Table
}

func NewGapSyncState(db database.Database, ...) {
grandpaDB := database.NewTable(db, gapSyncPrefix)
}

func (g *GapSyncState) Retrieve() (*GapSyncInfo, error) {...}

func (g *GapSyncState) Write(info *GapSyncInfo) error {
}
```

- for writing I would suggest to use scale.Marshal to encode the info into bytes
- no need to keep previous information, it is ok to replace the old info.