SOLR-17949: Add Azure Blob Storage backup repository module

[prateeksinghalgit]

Implements Azure Blob Storage backup repository as discussed in SOLR-17949.

Description

This PR adds a new backup repository implementation for Azure Blob Storage, enabling Solr collections to be backed up to and restored from Microsoft Azure.

Key Features:

• Full backup/restore functionality to Azure Blob Storage
• Support for 4 authentication methods (Connection String, Account Key, SAS Token, Azure Identity)
• Incremental backup support with versioning
• Data integrity verification (checksum validation)
• Compatible with Azurite emulator for local testing
• Comprehensive documentation and 76 passing unit tests

Solution

The implementation follows Solr's BackupRepository interface pattern, similar to existing S3 and GCS repository modules:

• BlobBackupRepository: Main class implementing Solr's BackupRepository interface
• BlobStorageClient: Wrapper for Azure SDK, providing file operations
• BlobIndexInput: Custom Lucene IndexInput for reading from Azure blobs
• BlobOutputStream: Custom output stream for writing to Azure blobs
• Authentication: Supports 4 methods via flexible configuration in solr.xml

All streaming operations are compatible with Solr's ResumableInputStream for fault-tolerant transfers.

Implementation stats:

• 8 implementation files (1,606 LOC)
• 8 test files (2,180 LOC)
• All dependencies Apache 2.0 licensed

Tests

Unit Tests: 76/76 passing (100%)

./gradlew :solr:modules:blob-repository:test
# Result: BUILD SUCCESSFUL - 76 test(s)

Test Coverage:

• Basic read/write operations
• Large file handling (1GB+)
• Binary data integrity
• Concurrent operations
• Stream lifecycle (close/resume behavior)
• Incremental backups
• All 4 authentication methods
• Integration with Azurite (local emulator)
• Integration with real Azure Blob Storage

Testing Instructions:
Can be tested locally with Azurite emulator (no Azure account needed) or with real Azure Blob Storage. See solr/modules/blob-repository/README.md for detailed setup instructions.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew :solr:modules:blob-repository:check (module-specific check passed).
• I have added tests for my changes.
• I have added documentation for the Reference Guide