chore: Pre-register a builtin OAuth2 application for the official Gitea mobile app (#38880)

This is a prepare and required step for upcoming Gitea Official Mobile
APP which supports login with OAuth2.

The official Gitea mobile app needs the same mechanism. This adds a
builtin application for it:

| | |
|---|---|
| client ID | `b757811a-05c8-4c76-8d74-a5ee3d2073f2` |
| config name | `gitea-app` |
| display name | `Gitea App` |
| redirect URI | `com.gitea.app://oauth/callback` |

Unlike the existing entries, which are CLIs and can therefore use a
loopback `http://127.0.0.1` redirect, a mobile app authorises through a
system browser session (`ASWebAuthenticationSession` on iOS, Custom Tabs
on Android) that can only receive a custom-scheme callback, hence the
custom scheme here.

modules/setting/oauth2.go

@@ -108,7 +108,7 @@ var OAuth2 = struct {
 	JWTSigningAlgorithm:        "RS256",
 	JWTSigningPrivateKeyFile:   "jwt/private.pem",
 	MaxTokenLength:             math.MaxInt16,
-	DefaultApplications:        []string{"git-credential-oauth", "git-credential-manager", "tea"},
+	DefaultApplications:        []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app"},
 }
 
 func loadOAuth2From(rootCfg ConfigProvider) {

modules/setting/oauth2_test.go

@@ -62,7 +62,7 @@ func TestGetGeneralSigningSecretSave(t *testing.T) {
 func TestOauth2DefaultApplications(t *testing.T) {
 	cfg, _ := NewConfigProviderFromData(``)
 	loadOAuth2From(cfg)
-	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea"}, OAuth2.DefaultApplications)
+	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app"}, OAuth2.DefaultApplications)
 
 	cfg, _ = NewConfigProviderFromData(`[oauth2]
 DEFAULT_APPLICATIONS = tea