[Fontations] Implement onGetTableTags()

Implement copying of table tags on the C++ side by calling a new FFI
method which iterates over TableDirectory.

Add basic test based on Ahem.

Bug: skia:14373
Change-Id: I05df55615a8027fcafe17a1d37fbcc16a9049c52
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/710796
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Dominik Röttsches <drott@google.com>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
4 files changed