Enhancement: Reduce Azure storage costs by optimizing upload method

Restic previously used Azure's PutBlock and PutBlockList APIs for all file
uploads, which resulted in two transactions per file and doubled the storage
operation costs. For backups with many pack files, this could lead to
significant Azure storage transaction fees.

Restic now uses the more efficient PutBlob API for files up to 256 MiB,
requiring only a single transaction per file. This reduces Azure storage
operation costs by approximately 50% for typical backup workloads. Files
larger than 256 MiB continue to use the block-based upload method as required
by Azure's API limits.

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