Add Subscription "Imports/Exports" to Settings/"Backup and restore" menu

[conkerts]

Checklist

• I made sure that there are no existing issues - open or closed - which I could contribute my information to.
• I'm aware that this is a request for NewPipe itself and that requests for adding a new service need to be made at NewPipeExtractor.
• I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.
• I have read the FAQ and my problem isn't listed.

Feature description

The Title basically sums it up, just add the Subscription Imports/Exports function to the Settings/Backup and restore menu, too.

Why do you want this feature?

I wanted to transfer my newpipe subscriptions from the phone to smarttube on the TV, but I ran into some issues.

I would have expected the export button for the subscriptions to be in the Settings/"Backup and restore" section,
instead I had to google it, and it was hidden in the hamburger menu of the subscriptions section. (imho it's also ok to have it there, but it was not the first place where I was searching, as I find having it in the Settings/"Backup and restore" section is more logical).

(On a side note: The newpipe faq mentions it, too https://newpipe.net/FAQ/tutorials/import-export-data/
tho the screenshots are a bit outdated ...)

Additional information

There was quite some discussion on that feature for smarttube, too:
yuliskov/SmartTube#4260

I still haven't found the import button there, but the dev said, it's going to get added to their settings/backups menu.

So I'd be nice to have these 2 things consistent across the apps.

[ba06]

I second this. I was trying to import subscriptions from FreeTube and the JSON file was grayed out when trying to import from the Settings section; only ZIP files were allowed. I had to find some obscure Reddit post that said it was in the Subscriptions tab in a three-dot menu. Pretty frustrating.

It shouldn't be too hard to either copy/paste from the Subscriptions import option or just link to it from the Backup/Restore menu.

[dev-victoria]

+1 we just had a case in the matrix chat(and it already happened to me too) where someone apparently exported his subscriptions to the json and tried to import them through the settings(as database), which obviously didn't work.

There's enough space, it should have a good description to avoid in place confusion tho.

Whoever is working at this(might be me later), might check how hard it would be to import the database's subscriptions for this page. (Additional warning dialog "only the subscriptions will be imported")

[AndrianaBilali]

Hi! I just wanted to follow up now that I’ve opened this PR.
I initially worked on this to bring the existing functionality (already available in the subscriptions tab) into the Settings > Backup & Restore screen for easier access.
I wasn't aware of the Matrix chat discussion when I started this, but I hope it’s a helpful first step toward reducing confusion around the different backup options.