Add preview Aspire nuget feed

nuget.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <packageSources>
        <clear />
        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
        <add key="aspire"
            value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
    </packageSources>

    <packageSourceMapping>
        <packageSource key="aspire">
            <package pattern="Aspire.*" />
        </packageSource>
        <packageSource key="nuget.org">
            <package pattern="*" />
        </packageSource>
    </packageSourceMapping>
</configuration>