Add shutdown attribute to Interfaces class and update related templates

- Added a `shutdown` boolean attribute to the `Interfaces` class to manage interface states.
- Updated the `eltex.ttp` template to include the `shutdown` command based on the new attribute.
- Modified expected configuration JSON files for various devices to reflect the new `shutdown` attribute, ensuring accurate testing and validation of interface configurations.

oxi/interfaces/contract.py

@@ -26,6 +26,7 @@ class Interfaces(Base):
     ip_address: IPv4Address | None = None
     mask: int | None = None
     description: str | None = None
+    shutdown: bool = False
 
 
 class Vlans(Base):