Added support for core auto-updates for registered sites

[dhanendran]

Description of the Change

Added support for plugin auto-update for registered user.

Possible Drawbacks

After updating to this version, this change might not be visible immediately as WP has already collected the data for existing plugins and stored it in transient for 12 hours (I guess). So either we need to manually delete the transient or wait for 12 hours.

Verification Process

• After checking out to this branch, manually delete the transient.
• _site_transient_update_plugins meta key name in sitemeta table needs to be deleted

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

#656

[dhanendran]

@dkotter using an update checker library is a great suggestion. I have updated my code to use them now.

[dkotter]

@dhanendran This looks a lot cleaner now, using that update class. I've left one minor comment but otherwise looks good to me code-wise. Have you tested this with the new approach and are seeing auto-updates work? I tried testing locally but I could never get auto-updates to fire, though may have just not waited long enough.

[dhanendran]

@dkotter looks like similar code already present in https://github.com/10up/distributor/blob/develop/distributor.php#L156-L188. Not sure why it is not working though, I will check the existing implementation first.

[jeffpaul]

@dhanendran any updates on your research on this topic?

[dhanendran]

@jeffpaul unfortunately no, as I was busy with other project priorities. I will try to check this in next couple of weeks.

[sksaju]

This PR is ready for your review @dkotter @jeffpaul