* Validate Azure storage account name format
For the commercial and government clouds the storage account name
becomes part of the service hostname, so enforce Azure's documented
format (3 to 24 lowercase letters and digits) both in config
validation and when building the service URL. Also treat an empty
AzureCloud as commercial, consistent with the URL builder.
* Route Azure custom endpoints through httpservice
The Azure custom-cloud backend now issues requests through
httpservice's transport rather than the SDK default, so they honor
ServiceSettings.AllowedUntrustedInternalConnections like other
outbound connections. The commercial and government clouds keep the
SDK default transport, which Private Link deployments rely on.
The allowlist value is plumbed into FileBackendSettings, and the
host/IP checks are factored into a shared httpservice helper that
MakeTransport now uses too.
