Add new request header property `req.last_event_id`

[vytas7]

Introduce a new request header property, req.last_event_id, as a nicer way to read out the value of the Last-Event-ID header.

It does not need to be cached since it would likely be read only once, and only for the SSE use case.

[Mubanga-W-Chashi]

Hello @vytas7
I'd like to assist with the above listed task. Kindly consider adding me to your team

[vytas7]

Hello @Mubanga-W-Chashi!
Sure, go ahead. Note that we don't assign anything per se, your comment here is enough for others to know you're working on it.