Add managed identity to connect an azblob

[Karreg]

Add managed identities as a way to connect azblob. More secure and simplier than other methods.

Tested and validated on an azure environment:

$ plakar store show
aztest:
    account_name: satestforplakar
    location: azblob://plakar
    passphrase: '********'
    use_managed_identity: "true"

# plakar at @aztest info
Version: v1.0.0
Timestamp: 2026-07-06 17:53:45.527958434 +0200 CEST
RepositoryID: c6f4e381-e63d-4d66-b398-3539721d0a75
Packfile:
 - MaxSize: 64 MiB (67108864 bytes)
Chunking:
 - Algorithm: fastcdc-v1.0.0
 - MinSize: 512 KiB (524288 bytes)
 - NormalSize: 1.0 MiB (1048576 bytes)
 - MaxSize: 8.0 MiB (8388608 bytes)
Hashing:
 - Algorithm: BLAKE3
 - Bits: 256
Compression:
 - Algorithm: LZ4
 - Level: 131072
Encryption:
 - SubkeyAlgorithm: AES256-KW
 - DataAlgorithm: AES256-GCM-SIV
 - ChunkSize: 65536
 - Canary: 140016d7a6cdecebe6ccf601aa7c76cb948c4eec2be2c73b89dc25d1104d382cefe974e0f479a5ecba1d16d1123c33dc62f7381b23d49c5f7870954ea39e14e1eaf21a67217c613735bb12083aa76cdc00436ae267131176b5032c56b41c1caf4109f71f
 - KDF: ARGON2ID
   - Salt: 5ee0641d270010138be1a6aff4a07aeb
   - SaltSize: 16
   - KeyLen: 32
   - Time: 4
   - Memory: 262144
   - Thread: 1
Snapshots: 11
Storage size: 16 EiB (18446744073709551615 bytes)
Logical size: 292 GiB (313723315920 bytes)