Add a new optional CLI leaf adjust-mss-direction under both
interfaces ... ip and interfaces ... ipv6.

Supported values:

• outbound (default, current behavior)
• inbound
• both

Implementation details:

• Extend MSS rule generation in Interface to support direction-aware rule matching (oifname for outbound, iifname for inbound, both for both).
• Keep backward compatibility by defaulting to outbound when direction is not configured.
• Improve MSS rule cleanup to remove both oifname and iifname rules for the interface, avoiding stale rules when direction changes.
• Wire direction config into interface update flow for IPv4 and IPv6 MSS paths.
• Extend interface smoketests to validate both creates inbound and outbound MSS rewrite rules for IPv4 and IPv6.

Files changed:

• /vyos-1x/interface-definitions/include/interface/adjust-mss.xml.i
• /vyos-1x/python/vyos/ifconfig/interface.py
• /vyos-1x/smoketest/scripts/cli/base_interfaces_test.py