Enhancement: Reduce Azure storage costs by optimizing uploads

Restic previously used Azure PutBlock and PutBlockList for every upload, which
cost two storage transactions per file and roughly doubled transaction charges
for repositories with many pack files.

Files up to 256 MiB now use PutBlob, requiring only a single transaction per
file and cutting typical transaction costs by about half. Larger blobs still
use block uploads as required by Azure.

https://github.com/restic/restic/issues/5531
https://github.com/restic/restic/pull/5544
