enhance(oauth): pre-register a builtin OAuth2 application for gitea-vscode (#39785) (#39801)

Backport #39785 by @bircni

The [Gitea VS Code extension](https://gitea.com/gitea/gitea-vscode)
currently signs in through the builtin `tea` OAuth2 application. Gitea
keeps one grant per user and application, so anyone who has already
signed in with the tea CLI gets `a grant exists with different scope`
when the extension requests its own scopes, and sign-in fails
(gitea/gitea-vscode#24).

This adds a dedicated builtin application for the extension, following
the same mechanism as the existing CLI entries:

| | |
|---|---|
| client ID | `75a9ec3e-42b5-40cb-9144-9ea96ecd56a6` |
| config name | `gitea-vscode` |
| display name | `gitea-vscode` |
| redirect URIs | `http://127.0.0.1`, `https://127.0.0.1` (loopback, any
port) |

It is enabled by default via `[oauth2].DEFAULT_APPLICATIONS`. Instances
that set that option explicitly need to add `gitea-vscode` to use it.

The extension will use this client on Gitea >= 28.2.0 and keep falling
back to `tea` on older servers, so this should be backported to v28.

Co-authored-by: bircni <bircni@icloud.com>

custom/conf/app.example.ini

@@ -638,7 +638,8 @@ ENABLED = true
 ;; * https://github.com/git-ecosystem/git-credential-manager
 ;; * https://gitea.com/gitea/tea
 ;; * Gitea App (the official Gitea mobile app)
-;DEFAULT_APPLICATIONS = git-credential-oauth, git-credential-manager, tea, gitea-app
+;; * https://gitea.com/gitea/gitea-vscode
+;DEFAULT_APPLICATIONS = git-credential-oauth, git-credential-manager, tea, gitea-app, gitea-vscode
 ;;
 ;; By default, OAuth2 applications can only use "http" and "https" as their redirect URI schemes.
 ;; If you need to use other schemes (e.g. for desktop applications), you can specify them here as a comma-separated list.