add github handle as additional userId

[sformel-usgs]

I have recently found myself googling dataset POCs to tag them in a github issue. It can be difficult sometimes to confirm that the github user is the same person listed as a dataset POC.

It would be great if a user's github handle could be included in the EML, as an additional userId. I would also view it's inclusion by a contact as implicit interest in being contacted this way.

[mike-podolskiy90]

Thank you for the suggestion