Enable strong-name signing

[stephentoub]

Enables other strong-name-signed netstandard2.0 assemblies to depend on Anthropic{.Foundry}.

Fixes #40

[sd-st]

We do want to implement strong-name signing, but internally we've discussed it and I don't believe we want to put the private key in the repo. We were planning to expose it as an environment variable within GitHub Actions, which we would then use for signing.

[ericstj]

We do recommend that folks make their key available for anyone to use. https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/strong-naming

In this case the Open.snk is the same that we make publicly available in https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.Arcade.Sdk/tools/snk/Open.snk. There's no good reason to keep it secret.

[rogerbarreto]

Initially I had second thoughts about exposing the Private/Public key as part of the snk file. But seems evident for the sake of Strong Name signing this is not an issue.

From my own experience regarding public/private key generation, normally the private is held secretly using the delayed signing approach, but OSS seems to not follow this case and I think is ok to moving forward as the PR suggest.