Add attachmentDirectoryName to AbstractAttachmentQueue get table and add blurb to Readme#523

jessemcintire wants to merge 3 commits intopowersync-ja:mainpowersync-ja/powersync-js:mainfrom

jessemcintire:feat/set-custom-attachments-table-namejessemcintire/powersync-js:feat/set-custom-attachments-table-nameCopy head branch name to clipboard