This is already there, just not documented in the spec.
This function would give you the exact location where to save a specific attachement according to the setting:
https://github.com/kometenstaub/linked-data-helper/blob/f0704cfda75e8a98ac29d52a7490a77053a41f8d/src/interfaces.ts#L26
You can also acess the raw settings value with:
this.app.vault.getConfig("attachmentFolderPath")
Using this value can be dangerous, the returned value might be a bit unexpected depending on what the setting is set to, better to use the other method if possible.