Add unmarshal json for template

[yoshinobc]

Motivation

Resolve a task #345
This is to add UnmarshalJSON functionality to TemplateMessage.

Description of the changes

• template_unmarshal.go file and created the UnmarshalTemplateJSON method. The logic of this method is almost identical to UnmarshalFlexMessageJSON method.
• Add test code to UnmarshalFlexMessageJSON (template_test.go)

What we considered but did not do

While implementing UnmarshalTemplateJSON, I noticed that UnmarshalJSON is not implemented for CarouselColumn and ImageCarouselColumn. However, considering the possibility that it might have a bad influence on other codes, we added rawColumn and rawImageColumn here and implemented UnmarshalJSON. If there is no risk of having a bad influence on other code, modify CarouselColumn and ImageCarouselColumn to add UnmarshalJSON.

What to do next

There was not enough testing for MarshalJSON in Template, so we will add it.

[codecov-commenter]

Codecov Report

Patch coverage: 82.14% and project coverage change: +0.15 🎉

Comparison is base (40ea0bd) 78.77% compared to head (89dfed9) 78.93%.

❗ Current head 89dfed9 differs from pull request most recent head 5035654. Consider uploading reports for the commit 5035654 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
+ Coverage   78.77%   78.93%   +0.15%     
==========================================
  Files          39       40       +1     
  Lines        3600     3712     +112     
==========================================
+ Hits         2836     2930      +94     
- Misses        581      590       +9     
- Partials      183      192       +9     

Impacted Files	Coverage Δ
linebot/template_unmarshal.go	82.14% <82.14%> (ø)

... and 1 file with indirect coverage changes

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[kkdai]

LGTM