Add fin and reset_stream fields

[LPardue]

Fixes #375.

Per the discussion, it can be useful to know when an application triggers a
stream close, or when it receives one, without having to do offset/length maths.

I omitted stop_sending in this commit because it seems a little bit of a stretch,
mainly because it's wire image has no offset or final size. Also, STOP_SENDING
relates to asking the peer to do something, which further complicates the
(potential) use of offset and length fields.

An alternative would be to keep fin here but define new events for reset and
stop. Those could both contain from, to and error fields but only the reset
would have offset and length.

[rmarx]

Discussed during editorial meeting:
Keep fin here, move reset_stream and stop_sending to separate event(s) (possibly having 2 because reset_stream is so different from stop_sending).

@marten-seemann it would help to have a quick mockup of what reset_stream_at might look like (not because we want to add it to main qlog yet, but if the current design would be compatible with that, it would be nice :)

[rmarx]

Actually discussed further during meeting, opinions changed :)

I'm now in favor of something quite general, say:

? additional_info: [+ text]

; possible values for QUIC/H3 currently are "fin_set", "stream_reset", "stream_reset_at", "stop_sending"

With the idea that other events can be used to determine what exactly happened (e.g., look at the actual STREAM_RESET frame) and keep this event what it was originally intended for: showing when which type of data (and now info) is communicated between the layers (as this doesn't always coincide with packet sent/received)