appservice: support to get external ip address used for DNS A records

[yugangw-msft]

fix #1836

[codecov-io]

Codecov Report

Merging #2627 into master will increase coverage by 0.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #2627      +/-   ##
==========================================
+ Coverage   71.87%   71.89%   +0.01%     
==========================================
  Files         377      377              
  Lines       20729    20745      +16     
  Branches     3048     3051       +3     
==========================================
+ Hits        14900    14914      +14     
- Misses       4890     4892       +2     
  Partials      939      939

Impacted Files	Coverage Δ
...vice/azure/cli/command_modules/appservice/_help.py	100% <100%> (ø)	⬆️
...e/azure/cli/command_modules/appservice/commands.py	89.7% <100%> (+0.15%)	⬆️
...ice/azure/cli/command_modules/appservice/custom.py	74.39% <85.71%> (+0.28%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 55705db...c3ee812. Read the comment docs.

[yugangw-msft]

Reopening. The PR was auto-closed when I rebased to the master and in the middle I removed my commit by mistake.

[yugangw-msft]

@derekbekoe , @troydai comparing with what you reviewed, I rolled back the command arg renaming, as I like to do that breaking change through a separate PR