[linkmgrd]: Add IPv6 only active-active link probing

[zjswhhh]

Why I did it

Active-active DualToR IPv6 only clusters can omit server_ipv4 and soc_ipv4. linkmgrd needs to keep IPv4 as the preferred path for dual-stack configs, but use IPv6 when IPv4 is absent.

How I did it

• Select server_ipv6 and soc_ipv6 only when the IPv4 field is missing.
• Add Loopback2/Loopback3 IPv6 config handling for link prober source addresses.
• Make software link prober packet layout, BPF filters, parsing, checksum, and GUID generation address-family aware.
• Publish IPv6 hardware ICMP echo session fields when the selected prober endpoint is IPv6.
• Add unit coverage for IPv6 fallback, ICMPv6 packet initialization, and hardware session publication.

How to verify it

• Run linkmgrd unit tests, including the new IPv6 fallback and ICMPv6 link prober cases.
• Build SONiC image with the corresponding buildimage submodule pointer update.
• On an IPv6 only active-active DualToR config, verify linkmgrd selects IPv6 only when IPv4 is absent and emits ICMPv6 probes.

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[zjswhhh]

CI dependency fix has been split into separate draft PR #345.

[mssonicbld]

/azp run

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