azureblob: Add support for custom upload headers

docs/content/azureblob.md

@@ -514,6 +514,18 @@ Properties:
 
 {{< rem autogenerated options stop >}}
 
+### Custom upload headers
+
+You can set custom upload headers with the `--header-upload` flag. 
+
+- Cache-Control
+- Content-Disposition
+- Content-Encoding
+- Content-Language
+- Content-Type
+
+Eg `--header-upload "Content-Type: text/potato"`
+
 ## Limitations
 
 MD5 sums are only uploaded with chunked files if the source has an MD5