StreamingDecoder: `DEC: BorrowMut<FrameDecoder>` isn't very useful

Currently, `DEC` must return a concrete instance of `FrameDecoder` for performing the actual decoding process. I couldn't customize its behavior to include functionality such as tracking the number of decoded bytes.

It would be helpful if `FrameDecoder` implemented a trait that wraps its `.read()` method. This way, `DEC` could be bound to that trait, allowing for more flexible and customizable behavior during decoding.

Start by locating DEC, FrameDecoder, and the decoding path that calls FrameDecoder.read(). Review how the concrete decoder is currently bound and determine the trait boundary needed for custom behavior such as byte tracking; done means existing decoding still works while DEC accepts compatible custom decoders.

Written by the indexing model from the issue text.