fix(vpp): backport portchannel punt support to 202605

[auspham]

Summary

Backport the PortChannel punt-via-member support from upstream PR #257 to the 202605 release branch.

Original PR: #257

Conflict resolution

The 202605 branch diverged before PR #255 introduced the sonic_ext plugin, so the three files modified by #257 were absent rather than relocated. The active custom-plugin source path remains vppbld/plugins, which vppbld/Makefile copies into the VPP source tree during a local build.

Instead of restoring only the three conflicted fragments, this backport adds the complete sonic_ext plugin tree exactly as it exists at the #257 merge commit and enables it in both VPP startup templates. This supplies the prerequisite plugin baseline and the PortChannel-specific behavior at the branch's active implementation path.

Validation

• git diff --check
• Confirmed no unresolved conflict markers
• Confirmed every added sonic_ext file is byte-for-byte identical to merge commit 53a0712b199b5cba54b6a433ca0b8c688d2c60cc
• make -s -n -C vppbld repo_clone
• Existing 202605 functional validation: [validation][202605] VPP PortChannel punt for DHCP relay sonic-buildimage#29067

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[auspham]

Added the missing PR #255 startup prerequisite in commit 1b24e83: disable linux-cp lcp-auto-subint in both VPP startup templates. The VLAN-BVI HLD requires this to keep tagged control-plane traffic on the physical member netdev where DHCP relay listens; leaving it enabled can create Ethernet<n>.<vid> LCP pairs that intercept the punted frame before VLAN delivery.

With this change plus #276, the sonic_ext plugin sources and VPP_VERSION=2606-0.4 are identical to the platform tree used by successful 202605 validation build 1196355 / Elastictest plan https://elastictest.org/scheduler/testplan/6a84bdd4e437251d82f52a1a.