Add support for multiple feeds

[bekir-ozturk]

Some of the archives for newer versions of .NET will only be released into a newly created blob storage.
To support both old scenarios and the new, the scripts should be able to acquire bits from multiple storage accounts.

• Modify both ps1 and sh scripts to attempt downloading from the new storage account in the case that the needed archives are not found in the old storage account.
• Only the sections related to downloading should be retried. Disk space checking, architecture detection etc shouldn't be retried with a new feed in case of a failure with the old feed.
• Errors happening on anywhere outside of downloading such as extraction, validation and cleaning up shouldn't cause a retry using the next feed.

This work will require some level of refactoring as the current structure of the scripts are not well enough to keep the maintainability high while adding this feature.