h264parse: Include coded-picture-structure info in caps

h264parse: Include coded-picture-structure info in caps

This reverts commit 652773de366a98e155332bf8ffb83ee5e6bea9e5 and
modifies it to rename the caps field name to coded-picture-structure.

It was previously removed because it confuses the decoder and we didn't
have a valid use case for including it in the encoded caps at this
stage. We now do have such a use case but still don't want to confuse
the decoder, so the field is renamed.

However, it is still not accurate without looking at the SEI picture
structure of each frame, so it was named coded-picture-structure. If its
value is "frame" it is most likely progressive, if it's "field" it is
most likely interlaced or mixed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2177>