Pre-register OAuth2 applications for git credential helpers (#26291)

Pre-register OAuth2 applications for git credential helpers (#26291)

This PR is an extended implementation of #25189 and builds upon the
proposal by @hickford in #25653, utilizing some ideas proposed
internally by @wxiaoguang.

Mainly, this PR consists of a mechanism to pre-register OAuth2
applications on startup, which can be enabled or disabled by modifying
the `[oauth2].DEFAULT_APPLICATIONS` parameter in app.ini. The OAuth2
applications registered this way are being marked as "locked" and
neither be deleted nor edited over UI to prevent confusing/unexpected
behavior. Instead, they're being removed if no longer enabled in config.

![grafik](https://github.com/go-gitea/gitea/assets/47871822/81a78b1c-4b68-40a7-9e99-c272ebb8f62e)

The implemented mechanism can also be used to pre-register other OAuth2
applications in the future, if wanted.

Co-authored-by: hickford <mirth.hickford@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
---------

Co-authored-by: M Hickford <mirth.hickford@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>