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, a single transaction per file, cutting
typical transaction costs about in 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
