sdp: stringify structured fields

sdp: stringify structured fields

The current parser implementations take a field, such as connection data, and
split it into subfields for a specific structure (e.g., struct ConnectionData).
However, following this approach requires several sticky buffers to match the
whole field, which can make a rule a bit verbose and doesn't offer any advantage
for matching specific parts of a field.

With this patch, a single line is still split into pieces if it makes sense for
parsing purposes, but these pieces are then reassembled into a single string.
This way, only one sticky buffer is needed to match the entire field.

Ticket #7291