Provide more convinient way how to extend EOL chroots

[nikromen]

I am pasting relevant conversation from #3974

Q: can you please consider a better way to opt-out of auto chroot expiration for projects that really do have a valid reason to keep EOL chroots around? It'd be nice not to have to resort to reverse engineering the form or get endless notification emails that require tedious manual button pressing.

A: This is a bit tricky subject. We generally avoid providing a permanent opt-out feature because we need to be somewhat agressive about pruning EOL chroots to conserve resources and prevent this from being misused. AFAIK this has been a recurring discussion, and unfortunately, we can't offer a simple auto opt-out as we need to manage storage space.

However instead that now you are trying to automate the action by reverse-engineering web form, we could perhaps provide official API endpoint for this action? @fedora-copr/copr-team WDYT about something like POST /api_3/copr_chroot//extend? It would at least a bit solve this user's problem and be much safer that scripting against the web forms.

Q: I wonder if allowing this to be disabled on a per-project basis is so bad. Is it really that likely that hoards of people are going to use this feature? In my case, I have several Coprs, some of which are ephemeral and have the delete after days feature enabled, the others which I just let the EOL chroots expire. I really only need this for the one specific project that only has one single package, and I'm already renewing those chroots manually, so a permanent opt-out wouldn't really change the storage usage situation at all in this case. Do you have data on how many people are already manually triggering chroot renewals? I doubt it's very many, but maybe I'm wrong. In any case, I have no decision making power here so feel free to ignore me :D.

Also user notes that the official POST endpoint would work.

cc @gotmax23