Add generic/windows-services.yml

[frack113]

Hello,
To keep up to date all the backend mapping for windows log is awful (#2553)
I would like to change to the actual way for windows rule to

windows-services.yml have only a logsources section and a order of 15
generic/powershell.yml is now in windows-services.yml (no more 2 powershell.yml 😄 )

powershell2.yml

title: Logsource to LogName mappings for PowerShell backend
order: 20
backends:
  - powershell
fieldmappings:
    Channel: LogName

winlogbeat-modules-enabled2.yml

title: Elastic Winlogbeat (from 7.x) index pattern and field mapping following Elastic enabled Modules
order: 20
backends:
  - es-qs
  - es-dsl
  - es-rule
  - es-rule-eql
  - es-eql
  - kibana
  - kibana-ndjson
  - xpack-watcher
  - elastalert
  - elastalert-dsl
  - ee-outliers
defaultindex: winlogbeat-*
fieldmappings:
    EventID: event.code
...