[iOS] Create Provider Function For Passing Image Attachments To Floaty

This API allows GeminiBrowserAgent to pass an image attachment to the
floaty without resetting the floaty instance when Copresence is
enabled.

No-op until crrev.com/i/9005696 lands.

Bug: 482334416
Change-Id: I2f8e1bc51454df11dd9a63c81ace8ed1cee35c80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7559056
Reviewed-by: Adam Arcaro <adamta@google.com>
Commit-Queue: Joemer Ramos <joemerramos@google.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Cr-Commit-Position: refs/heads/main@{#1581923}