[Enhancement] API ``/api/v1/videos/:id`` - Return ``published`` for ``recommendedVideos``

[litetex]

Is your enhancement request related to a problem? Please describe.
I'm currently implementing Invidious support into NewPipe (TeamNewPipe/NewPipe#8220) and noticed that there is no published-date/timestamp for related/recommended videos.

Describe the solution you'd like
The timestamp should be exposed via the API.

Describe alternatives you've considered
Currently you have to switch to the NewPipe-native YT extraction to see the publish-date/time of a related video.

Additional context
N/A

[RadoslavL]

I am currently working on this on my fork. What format would you prefer for the published timestamp? The full time, just the date or in the Unix time format?

Edit: I ended up using the Unix time format in the PR, but I can still change it, if you think it's not the best option.