### Describe Your Changes
These changes support using Azure Managed Identity for the `vmbackup`
utility. It adds two new environment variables:
* `AZURE_USE_DEFAULT_CREDENTIAL`: Instructs the `vmbackup` utility to
build a connection using the [Azure Default
Credential](https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/sdk/azidentity@v1.5.2#NewDefaultAzureCredential)
mode. This causes the Azure SDK to check for a variety of environment
variables to try and make a connection. By default, it tries to use
managed identity if that is set up.
This will close
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5984
### Checklist
The following checks are **mandatory**:
- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
### Testing
However you normally test the `vmbackup` utility using Azure Blob should
continue to work without any changes. The set up for that is environment
specific and not listed out here.
Once regression testing has been done you can set up [Azure Managed
Identity](https://learn.microsoft.com/en-us/entra/identity/managed-identities-azure-resources/overview)
so your resource (AKS, VM, etc), can use that credential method. Once it
is set up, update your environment variables according to the updated
documentation.
I added unit tests to the `FS.Init` function, then made my changes, then
updated the unit tests to capture the new branches.
I tested this in our environment, but with SAS token auth and managed
identity and it works as expected.
---------
Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>
Co-authored-by: Justin Rush <jarush@epic.com>
Co-authored-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>
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