Add InputOption for RichMenuAction

[s-aska]

It seems that the InoutOption of PostbackAction is missing from RichMenuAction.

official doc: https://developers.line.biz/ja/reference/messaging-api/#postback-action
ref: https://github.com/line/line-bot-sdk-go/blob/master/linebot/actions.go#L113

I'm not very experienced, so please let me know if there's anything amiss or if I've overlooked something.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (df06aa0) 78.93% compared to head (0d8de23) 78.93%.
Report is 1 commits behind head on master.

❗ Current head 0d8de23 differs from pull request most recent head aadd908. Consider uploading reports for the commit aadd908 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #358   +/-   ##
=======================================
  Coverage   78.93%   78.93%           
=======================================
  Files          40       40           
  Lines        3712     3712           
=======================================
  Hits         2930     2930           
  Misses        590      590           
  Partials      192      192           

Files Changed	Coverage Δ
linebot/richmenu.go	62.60% <ø> (ø)

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[Yang-33]

Thank you!

[kkdai]

LGTM