Optionally show publicly whether user has enabled 2FA

[rugk]

It would be great if one could opt-in to show publicly that the user has enabled "2FA" for their account.

For users downloading crates, this can ensure confidence and promote the very important security features.

[carols10cents]

There is no 2FA on crates.io; we use GitHub for authentication and as far as I can tell, 2FA enablement is not something available through GitHub's API.

Furthermore, GitHub might not always be the only authentication provider. We'd have to make sure every provider made 2FA status available through their API... so I don't think it's likely we'd implement this.