Add T1087 attack technique (account discovery)

[shreyamalviya]

Fixes #705

Some things to address:

• Should we include more information such as the groups that a user belongs to, for Linux? Right now it just displays the user account names.
• In Windows, the command to list out users is net user which outputs a bunch of other stuff, which is a little hard to manipulate. And we can't use the content from dir C:\Users\ /b since a user's folder is only created when the user logs in. Should we show the whole output content of net user in the report?
  

[codecov]

Codecov Report

Merging #793 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #793   +/-   ##
========================================
  Coverage    60.33%   60.34%           
========================================
  Files          161      161           
  Lines         4901     4902    +1     
========================================
+ Hits          2957     2958    +1     
  Misses        1944     1944           

Impacted Files	Coverage Δ
...s/config_schema/definitions/post_breach_actions.py	100.00% <ø> (ø)
.../monkey_island/cc/services/config_schema/monkey.py	100.00% <ø> (ø)
monkey/common/data/post_breach_consts.py	100.00% <100.00%> (ø)

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[ShayNehmad]

Re including groups: not strictly necessary (but useful if it's cheap).
Re net user: powershell "Get-LocalUser" will work (and look) better. You could pipe the results into format-list or format-table or even powershell "Get-LocalUser | ConvertTo-Json".