[Feature request] Request the ability to `sync` single component of chain

As I describe in [here](https://stackoverflow.com/questions/67268620/how-to-only-flushsync-one-component-of-boost-filtering-stream). I need flush only the first component of chain, but `sync_impl` method of indirect_streambuf is private. I can't think of a better way to achieve this.

Start with the indirect_streambuf entry point and its private sync_impl method, then read the linked Stack Overflow example to understand the single-component synchronization need. Completion should provide a clearly supported way to sync only the first chain component and demonstrate that behavior without relying on the private method.

Written by the indexing model from the issue text.