[Nexthop] Skip setting CREATE_ONLY next hop group attributes

[daria-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

The HW agent SIGABRTs on warm boot in AgentFlowletSprayTest.VerifyEcmpRandomSpray and AgentLoadBalancerTestV6EcmpSpray.EcmpLoadBalanceFullHashCpuTraffic on Tomahawk6 platforms. SAI_NEXT_HOP_GROUP_ATTR_HIERARCHICAL_NEXTHOP is CREATE_ONLY in the SAI spec, and Broadcom 14.2 SAI doesn't implement a getter. On warm boot SaiStore::reload() falls back to the attribute's StdNullOptDefault, and SaiObject::checkAndSetAttribute sees nullopt != false and issues a set on a create only attribute, which returns NOT_SUPPORTED and hw agent aborts.

This change adds an IsCreateOnlyAttribute trait, specialised for HierarchicalNextHop. checkAndSetAttribute() no longer writes create-only attributes to hardware; instead, the store adopts the desired value in its cache, preserving the value programmed during object creation.

HashAlgorithm is included on spec grounds: it is also CREATE_ONLY, but it does not trigger this issue because Broadcom SAI implements its getter.

Test Plan

The tests now pass:

[ PASSED ] cold_boot.AgentFlowletSprayTest.VerifyEcmpRandomSpray (39999 ms)
[ PASSED ] warm_boot.AgentFlowletSprayTest.VerifyEcmpRandomSpray (37260 ms)
[ PASSED ] cold_boot.AgentLoadBalancerTestV6EcmpSpray.EcmpLoadBalanceFullHashCpuTraffic (21232 ms)
[ PASSED ] warm_boot.AgentLoadBalancerTestV6EcmpSpray.EcmpLoadBalanceFullHashCpuTraffic (19543 ms)