[storage-blob-preview] `az storage blob download`: Allow downloading to stdout for pipe support

[evelyn-ys]

Feature requested by https://github.com/Azure/azure-cli/issues/16925 --file used to be required as local file path for az storage blob download. This PR allows downloading blob content to stdout for pipe support when no --file specified.

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[yonzhan]

storage-blob-preview

[bittrance]

Ran into this use case today - it would be nice if this PR could be finalized and merged.