Introduce a base() method to get the baseExecutor, so we don't need to add
method from baseExecutor to the Executor interface any more
Before:
```
type Executor interface {
Open(context.Context) error
Next(ctx context.Context, req *chunk.RecordBatch) error
Close() error
Schema() *expression.Schema
retTypes() []*types.FieldType
newFirstChunk() *chunk.Chunk
}
```
After:
```
type Executor interface {
base() *baseExecutor
Open(context.Context) error
Next(ctx context.Context, req *chunk.RecordBatch) error
Close() error
Schema() *expression.Schema
}
```
