Use DefaultAzureCredential for managed identity in azure blob extention

[te-chan2]

Summary

I prefer not to save the key as an environment variable when using Dify on Azure. I would like to modify it so that when AZURE_BLOB_ACCOUNT_KEY is set to managedidentity, it uses DefaultAzureCredential to use the Managed Identity assigned to the resource.

Resolves #11558

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods