firewall: T3509: Add support for IPv6 reverse path filtering

[sarthurdev]

Change Summary

Moves reverse path filtering to nftables with added support for IPv6.

• Add set interfaces ethernet ethN ipv6 source-validation for per-interface setting.
• set firewall global-options source-validation will set it across all interfaces, for both IPv4 and IPv6.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T3509

Component(s) name

firewall

Proposed changes

• Moves reverse path filtering handling into nftables for both IPv4 and IPv6 support and removes sysfs references.
• Adds smoketest to verify rules

How to test

Testing pending, marked ready once tested.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[c-po]

Can we please make this "per interface" as we do with IPv4 and e.g. MSS settings?

set interfaces ethernet eth2 ipv6 source-validation