Implement expiry for stitcher to make sessions long lived

[matvp91]

Is your feature request related to a problem? Please describe.
When stitcher creates a session, it'll expire after a couple of minutes. When we report a bug elsewhere and want to include a playlist generated by stitcher, we now have to paste in cURL commands and I'd much rather provide a direct playlist link (eg, video-dev/hls.js#6768 (comment)).

Describe the solution you'd like
Implement expiry so we can manually set the expiration time for each session.

When we share public playlists generated by stitcher, we can set the expiry to a couple of months and have them available for much longer.