Allow multiple feedConfigs per category when publishing from manifests

[riarenas]

This is the first of a few PRs that need to go in sequentially in order to enable #3607 and #3605

A validation build that published the Installer and BinaryLayout categories to two locations can be found here: https://dev.azure.com/dnceng/internal/_build/results?buildId=312203&view=results

Specifically: https://dev.azure.com/dnceng/internal/_build/results?buildId=312203&view=logs&j=841c9107-1508-5744-5f38-ba9581e84f26&t=3d448e79-a526-559e-582a-2d42509f1233&l=52

Artifact with category 'NetCore' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' 
| Artifact with category 'OSX' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'Deb' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'Rpm' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'Node' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'BinaryLayout' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'Installer' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'Checksum' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'Maven' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'VSIX' should go to AzureStorageFeed -> 'https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json' |  
  | Artifact with category 'BinaryLayout' should go to AzureStorageFeed -> 'https://dotnetcli.blob.core.windows.net/riarenas/index.json' |  
  | Artifact with category 'Installer' should go to AzureStorageFeed -> 'https://dotnetcli.blob.core.windows.net/riarenas/index.json' |  
  | Artifact with category 'VSIX' should go to AzureStorageFeed -> 'https://dotnetcli.blob.core.windows.net/riarenas/index.json'

Makes it easy enough to set up multiple TargetFeedConfigs:

 <ItemGroup Condition="'$(TargetFeedConfig)' == ''">
      <TargetFeedConfig
        Include="NetCore;OSX;Deb;Rpm;Node;BinaryLayout;Installer;Checksum;Maven;VSIX"
        TargetURL="$(TargetStaticFeed)"
        Type="AzureStorageFeed"
        Token="$(AzureStorageTargetFeedPAT)" />
      <TargetFeedConfig
        Include="BinaryLayout;Installer;VSIX"
        TargetURL="$(InstallersTargetStaticFeed)"
        Type="AzureStorageFeed"
        Token="$(InstallersAzureAccountKey)" />
</ItemGroup>

[JohnTortugo]

I'll merge it to speed up things.