[Package Signing] Enable signature registration upon package upload

[anangaur]

/cc: @rido-min @onovotny (your feedback)

Package upload

Note: The 2FA requirement for registering certificates applies.

Package push (P2)

With an additional push option:

> dotnet nuget push <package> --register-certificate

[rido-min]

When receiving a valid signed package, I would register the certificate always, without requiring an explicit gesture (checkbox in Web, --register-certificate in CLI).

The signature probes the user has access to the private key, and that should be enough.
Registering a certificate ahead of time can be considered as Opt-In for extra auth protection, but not a requirement.