[Feature] Allow async events to be registered from LUA rules

[Feature] Allow async events to be registered from LUA rules

Here is an example:

```lua

rspamd_config.ASYNC_RULE = {
  callback = function(task)
    local ret = false

    local function test1(task)
      local function test2(task)
        return ret
      end

      ret = true
      return test2
    end

    return test1
  end
}
```