Packet passthrough support

[materight]

🚀 The feature

Allow to re-mux packets to a new container without decoding and re-encoding. Initial support for it seems to already be there (#3220) but it's currently not usable in python.

Motivation, pitch

Allowing to re-mux packets to a container would make more some processing more efficitent if only a part of the source streams need to be transformed (e.g. when modifying a video while keeping the original audio).

Alternatives

No response

Additional context

No response