Support user account_host and check if blob size exists

[KeerthiYandaOS]

• Adlfs spec has the hostname with "blob.core.windows.net" but all the URLs that we access might not have the same extension in the host, it will be helpful if we can provide an ability to accept the URL host from the user.
• We are checking the blob size here but we are seeing the error as the "size" parameter is not found. In order to overcome this, we can check if the size attribute exists and then check the size value.

[KeerthiYandaOS]

Created pull request for this issue: https://github.com/fsspec/adlfs/pull/341