feat: add Azure Blob Storage backend support

[GavinPJK]

Changes

• Create azblob backend type, using the Azure SDK for Go
• Add Azure-specific environment variables (AZBLOB_, AZURE_) and flags (azblob-container and azblob-prefix)
• Create integration test at integrationtests/integration_azblob_test.go
• Create GitHub actions example at examples/github_actions_azblob.yml
• Add new backend details to the README

Context

This PR adds Azure Blob Storage (azblob) as a backend option, implemented in a similar way to GCS and S3 (#2).

Authentication

The client supports three auth methods, attempted in the recommended order-of-preference - Entra ID/SAS Token are both tried before any shared connection strings

Choice of Client

Streaming methods from Azure SDK's high-level azblob.Client are used here:

• UploadStream takes any io.Reader and doesn't need the total size up-front
• The alternatives (UploadBuffer/UploadFile) require the whole object as an in-memory []byte or a seekable *os.File

Performance

Azure's closest analogue to S3OZ is a Premium Block Blob Storage account with in-region Local Redundant Storage, which the user accesses over a private endpoint (this has been added to the README)

[GavinPJK]

@richardartoul just wondering if you are available to assess this?

[richardartoul]

thanks @GavinPJK