Clone media metadata (Fixes #1280)

[dac514]

Fix is different than what #1280 recommends. Reasons:

• If a book is public then wp/v2/media exposes all media in the library. No concept of public/private per image.
• A user could upload 100 images but their book only uses 1 image.
• _wp_attachment_metadata can be fake or incompatible. It's more accurate to side load the file and have WordPress recreate this kind of info.

Fix keeps the current approach of looking for media actually being used in book, then copies PB metadata when found.

[codecov]

Codecov Report

Merging #1337 into dev will increase coverage by 0.01%.
The diff coverage is 90.47%.

@@             Coverage Diff              @@
##                dev    #1337      +/-   ##
============================================
+ Coverage     61.87%   61.88%   +0.01%     
- Complexity     4174     4179       +5     
============================================
  Files           112      112              
  Lines         18212    18230      +18     
============================================
+ Hits          11268    11282      +14     
- Misses         6944     6948       +4