soft_delete: support extra fields

[qqxhb]

Allow updating additional columns in the same soft-delete SQL.

Usage:

• Tag: gorm:"softDelete:flag,ExtraFields:DeletedBy" (multiple names separated by ; or |)
• Pass values via: db.Set(soft_delete.ExtraSettingsKey, map[string]interface{}{"DeletedBy": userID})
• In hooks: soft_delete.SetExtra(tx, map[string]interface{}{"DeletedBy": userID})

Notes:

• ExtraSettingsKey is consumed per statement to avoid leaking values across operations.

Tests:

• go test ./...