azure: let the blob endpoint be configured (#10460)

* azure: let the blob endpoint be configured

The service url was always derived as <account>.blob.core.windows.net,
which leaves out Azure Government, Azure China, and private endpoints.
Name the blob service url instead and those accounts become reachable.
The url has to be https, since the account key or the bearer token would
otherwise travel in the clear.

* azure: reject an endpoint that carries no hostname

A url like https://:443/ has a host of ":443", so the emptiness check on
Host let it through and the request only failed once it reached Azure.
The hostname is what has to be there.

weed/command/scaffold/replication.toml

@@ -67,6 +67,9 @@ account_key = ""
 # takes its tenant and token from AZURE_TENANT_ID and AZURE_FEDERATED_TOKEN_FILE,
 # which the Azure workload identity webhook projects into the pod.
 client_id = ""
+# blob service url, for accounts outside the public cloud. Empty resolves to
+# https://<account_name>.blob.core.windows.net/
+endpoint = ""
 container = "mycontainer"      # an existing container
 directory = "/"                # destination directory
 is_incremental = false