Add API for get-bot-info

[hinoguma]

Update Messaging API October 2020
https://developers.line.biz/en/news/2020/10/06/messaging-api-update-october-2020/

• Get bot info
• Get webhook endpoint information
• Set webhook endpoint URL
• Test webhook endpoint

[codecov-io]

Codecov Report

Merging #232 into master will decrease coverage by 0.89%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
- Coverage   77.59%   76.70%   -0.90%     
==========================================
  Files          33       34       +1     
  Lines        2415     2086     -329     
==========================================
- Hits         1874     1600     -274     
+ Misses        436      380      -56     
- Partials      105      106       +1     

Impacted Files	Coverage Δ
linebot/client.go	68.00% <ø> (-5.34%)	⬇️
linebot/response.go	66.11% <71.42%> (-3.94%)	⬇️
linebot/get_bot_info.go	100.00% <100.00%> (ø)
linebot/httphandler/httphandler.go	71.42% <0.00%> (-5.50%)	⬇️
linebot/webhook.go	71.42% <0.00%> (-3.58%)	⬇️
linebot/get_ids.go	78.31% <0.00%> (-2.94%)	⬇️
linebot/demographic_filter.go	85.00% <0.00%> (-2.76%)	⬇️
linebot/flex_unmarshal.go	66.37% <0.00%> (-2.49%)	⬇️
... and 27 more

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[kkdai]

Thank you for your PR, I have reviewed your PR.
Could you help to check for "displayName"

[hinoguma]

Thank you for review.

Thank you for your PR, I have reviewed your PR.
Could you help to check for "displayName"

I fixed missing displayName.
Could you please review again ?

[kkdai]

Thank you, you almost finish with this PR.
Please help to fix the final part.

[hinoguma]

Oh, sorry... I fixed and please check it.

[kkdai]

Thank you for your hard work, it looks good to me.
Looking forward to seeing you for the rest of the API.

[kkdai]

LGTM