Add ability to have multiple active affiliations per github user

[evrardjp]

Hello,

I am a consultant and I work for multiple companies.
As of today, the structure of the gitdm developer affiliations force the users to create a github account for each affiliation.

I think it would be best to be able to map the emails or domains to an affiliation. In my case, jean-philippe.evrard@customer{1,2,3}.com could be mapped to affiliations customer1, customer2, customer3 respectively. And all those emails would work with my sufficiently secured github account that I use for other CNCF needs.

Does that make sense?

[evrardjp]

@caniszczyk WDYT of this?