[pdf] Add AttributeList::appendTextString

Bug: 467952961
Change-Id: I3e6605e2d805606e45b9b079fe42c8c89fddd9cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1123456
Auto-Submit: Ben Wagner <bungeman@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>