Add --ignore-duplicate flag to avoid build failures on repeated CI builds

[bbourdrez]

When doing builds from github actions or other CI/CD systems it's often good to not fail on repeated production builds of the same package, so this new flag allows you to tell gpr to succeed when pushing a duplicate version (other than showing the server-provided error in the output).
There doesn't seem to be any mocking set up for the network-related parts of the code, so there's no tests.

[f2calv]

The official argument is --skip-duplicate, so I would be inclined to rename --ignore-duplicate to match;

• https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-nuget-push

[alexrp]

@jcansdale @bbourdrez any way we can move this one forward? I've found myself needing this a few times now.