Modify Microsoft.SymbolUploader.Build.Task to support uploading to multiple endpoints

[Mike McLaughlin]

@mikem8361 commented on Tue May 15 2018

Since we upload to both msdl and symweb now, it would be more efficient to modify the symbol uploader build task to accept multiple symbol server URLs and PATs. The PAT would have to become a metadata on the SymbolServer TaskItem. This would allow the files to be opened, read and any Portable PDB to Windows conversion to happen only once.

The Microsoft.Symbol.Uploader internal PublishFileInfo lifetime logic would have to change not to Dispose after the file is uploaded.