Enable Certificate authentication option in API template

[mkArtakMSFT]

This is listed in the Templating epic with questionmark, so not sure what exactly needs to happen here or how important it is to get it done in 3.0.

There is also tooling impact here. There is no such option in the New Project Dialog UI

[mkArtakMSFT]

This depends on https://github.com/aspnet/AspNetCore/issues/4663

[blowdart]

Given the desire to drive templates to be smaller, this is probably a rare enough want that it doesn't need template integration, but documentation will suffice.

[analogrelay]

Agreed. I don't think we should overload the template with samples. We just finished scaling it back :)