Add support for writing to a generic section stream

[jensenn]

Thanks for all your review of my pull requests!

I need a way to write SCTE-35 splice data into a TS stream. At this time, reading is not necessary.

SCTE-35 splice_info_section is a short format section stream that normally only spans a single TS packet, but could span multiple packets. The first packet needs the PUSI set and inserts a pointer_field byte. Subsequent packets do not set PUSI and do not have the pointer_field.

TsPayload::Raw will work for subsequent packets, but right now, there is not an appropriate TsPayload to use for the first payload. It really is a PSI table, but I have a pile of bytes that I don't want to have to parse into a PsiTable. Even then, there isn't a TsPayload to write a generic Psi.

Here is my proposal, but I don't know if it is the right thing to do. It is kind of a very low-level interface for generic section streams. I welcome your feedback.

Again, this is just writing. Reading would take a pretty large change to the reader as it assumes all elementary streams in the PMT are PES and doesn't handle section streams. Maybe we can think about that for another day.

[sile]

Thank you as always!
I'll review this PR within a week.

[sile]

I'm not familiar with SCTE-35, but your approach looks reasonable 👍

[sile]

About the reading part, let's discuss how to implement that when it becomes necessary.