Add validating message objects APIs

[Yang-33]

Resolve #327

[codecov-commenter]

Codecov Report

Base: 79.00% // Head: 78.69% // Decreases project coverage by -0.30% ⚠️

Coverage data is based on head (879ef0f) compared to base (c173a44).
Patch coverage: 69.56% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
- Coverage   79.00%   78.69%   -0.31%     
==========================================
  Files          38       39       +1     
  Lines        3443     3558     +115     
==========================================
+ Hits         2720     2800      +80     
- Misses        553      578      +25     
- Partials      170      180      +10     

Impacted Files	Coverage Δ
linebot/client.go	69.46% <ø> (ø)
linebot/validate_message.go	69.56% <69.56%> (ø)

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[kkdai]

LGTM