Azure Blob as a storage backend

### Background

We are interested in running Cachew in Azure environments where Azure Blob Storage is the preferred shared object store.

Cachew currently supports S3 for remote cache and metadata storage. We would like to understand whether the project would be open to adding native Azure Blob Storage support for:

- A cache backend.
- A metadata backend shared across Cachew replicas.

A possible configuration could look like:

```hcl
azure-blob {
account-url = "https://account.blob.core.windows.net"
}

cache azure-blob {
container = "cachew"
max-ttl = "8h"
}

metadata azure-blob {
container = "cachew"
}
```

Authentication would use the standard Azure SDK credential mechanisms, with connection-string support for local development and testing.

Before preparing a contribution, we would appreciate feedback on:

- Whether native Azure Blob Storage support fits the project's direction.
- Whether both cache and metadata backends would be welcome.
- Any preferred design constraints or consistency requirements.
- Whether this should use the existing backend interfaces directly or follow another planned storage abstraction.

If the maintainers are open to this, we can prepare a more detailed proposal and implementation.