Add Amka number (Greek social security number) to Extensions.Greece

[sevdalone]

Please describe why you are requesting a feature

Add Greek Amka generation
http://www.amka.gr/tieinai_en.html

Please provide a code example of what you are trying to achieve

I have pull request ready to be reviewed however I don't have access to create one yet. Please advise

(Please be complete. Include all code necessary to run your example in LINQPad.)
(The more complete code examples are, the more accurate answers will be.)
(https://www.linqpad.net)

Please answer any or all of the questions below

• Is the feature something that currently cannot be done?
  No

• What alternatives have you considered?
  None

• Has the feature been requested in the past?
  No

If the feature request is approved, would you be willing to submit a PR?
Yes

Yes / No (Help can be provided if you need assistance submitting a PR)