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

### 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.)_
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### 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)_

Start by reviewing the Extensions.Greece entry point named in the issue and the AMKA specification at amka.gr. Done means Greek AMKA generation is available there and follows the requested specification; the issue author says a pull request is already ready for review.

Written by the indexing model from the issue text.