Switch to NuGet trusted publishing

[martincostello]

We should switch from using API keys to publish packages to NuGet.org to the new Trusted Publishing feature: New Trusted Publishing enhances security on NuGet.org

This is more secure, and avoids the need to renew/rotate NuGet API keys.

See grafana/grafana-opentelemetry-dotnet#266 for an example of adopting this.

As you have three different GitHub Actions workflows to publish the NuGet packages, you'll need to create a trusted publishing policy for each one. Values you'll need are below:

• Repository organization: grafana
• Repository name: pyroscope-dotnet
• Workflow file: tag_managed_helper.yml or tag_tracing_opentracing_helper.yml or tag_tracing_opentelemetry_helper.yml