Support imageSet fields in webhook event

[clsung]

• fixes Support imageSet fields in webhook event #288

Ref: https://developers.line.biz/en/news/2021/09/07/image-set-has-been-added-to-webhook/

[codecov-commenter]

Codecov Report

Merging #291 (630672c) into master (089118a) will decrease coverage by 0.10%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
- Coverage   77.94%   77.83%   -0.11%     
==========================================
  Files          36       37       +1     
  Lines        2847     2856       +9     
==========================================
+ Hits         2219     2223       +4     
- Misses        497      502       +5     
  Partials      131      131              

Impacted Files	Coverage Δ
linebot/imageset.go	0.00% <0.00%> (ø)
linebot/event.go	88.95% <100.00%> (+0.12%)	⬆️
linebot/message.go	78.67% <100.00%> (+0.15%)	⬆️

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[kkdai]

LGTM