sout: add `set_pcr` callback

This is the first step for the `SetPCR` implementation in stream output.
These callbacks will allow stream outputs to have precise information
about the stream time advancement.

This patchset only provides the basis for now, a correct implementation
for all stream filters will come in a separate merge request.

Refs #27050

Co-authored-by: Alexandre Janniaux <ajanni@videolabs.io>