Feature: Add the support to use thumbnail size for querying Photos Asset

[dreampiggy]

The Thumbnail Decoding feature is introduced in SDWebImage 5.5.0: https://github.com/SDWebImage/SDWebImage/wiki/Advanced-Usage#thumbnail-decoding-550

For Photos URL, unify the API can make it easy to use for some of users.

[codecov]

Codecov Report

Merging #15 into master will decrease coverage by 0.55%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
- Coverage   62.40%   61.85%   -0.56%     
==========================================
  Files           3        3              
  Lines         258      270      +12     
==========================================
+ Hits          161      167       +6     
- Misses         97      103       +6     

Flag	Coverage Δ
#iOS	61.85% <40.00%> (-0.56%)	⬇️

Impacted Files	Coverage Δ
...WebImagePhotosPlugin/Classes/SDImagePhotosLoader.m	58.10% <40.00%> (-0.47%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 063852b...46b9ad9. Read the comment docs.