feat: mysql-bkup v2 on bkupkit

MysQL backup and restore built on the shared bkupkit core:
streaming end to end, with no dump held in memory or written to
local disk.

docs/how-tos/azure-blob.md

@@ -48,7 +48,7 @@ services:
       - DB_PORT=3306
       - DB_HOST=mysql
       - DB_NAME=database
-      - DB_USERNAME=username
+      - DB_USER=username
       - DB_PASSWORD=password
       ## Azure Blob Configuration
       - AZURE_STORAGE_CONTAINER_NAME=backup-container