Support append_position to avoid duplicate log

[Binyang2014]

According to https://docs.microsoft.com/en-us/rest/api/storageservices/append-block#avoiding-duplicate-or-delayed-appends.
We can avoid duplicate logs by using x-ms-blob-condition-appendpos.
But current implement seems not support that option:

fluent-plugin-azure-storage-append-blob/lib/fluent/plugin/out_azure-storage-append-blob.rb

Line 206 in 8736061

@bs.append_blob_block(@azure_container, @azure_storage_path, content[position..position + size])

Can we support this option so do not need to handle duplicate log case