[Nexthop][NPI] "asic_config_v3" - Add ladakh800bcls platform and its variants

[marif-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

Implements asic_config_v3 design.

In this PR, add platform configurations for the rack and test_fixture variants of ladakh800bcls platform.

After this commit the asic_config_v3 generator produces the following extra output files

• fboss/lib/asic_config_v3/generated_asic_configs/ladakh800bcls_rack.yml
• fboss/lib/asic_config_v3/generated_asic_configs/ladakh800bcls_test_fixture.yml

Test Plan

The generated output files are compared against the asic_config_v2 synced configs. Both variants show the same small set of differences:

$ diff fboss/lib/asic_config_v3/generated_asic_configs/ladakh800bcls_rack.yml fboss/lib/asic_config_v2/synced_asic_configs/ladakh800bcls_rack.yml
1932d1931
<       sai_stats_disable_mask: 0x800
1933a1933
>       sai_stats_disable_mask: 0x800
1941a1942,1944
>       l3_ecmp_member_secondary_mem_size: 8192
>       l3_ecmp_member_first_lkup_mem_size: 8192
>       dlb_quality_adjustment_default_value_select: 1
2016a2020,2025
> ...
> ---
> device:
>   0:
>     DLB_ECMP_CONFIG:
>       PATH_QUALITY_PROFILE_MODE: WIDE

$ diff fboss/lib/asic_config_v3/generated_asic_configs/ladakh800bcls_test_fixture.yml fboss/lib/asic_config_v2/synced_asic_configs/ladakh800bcls_test_fixture.yml
<same as above>

The differences are limited to ordering of keys in the global device table (the SDK does not depend on this order) and a few keys (l3_ecmp_member_*, dlb_quality_adjustment_default_value_select, and the DLB_ECMP_CONFIG block) that are only present in the stale asic_config_v2 synced configs. These differences are expected.

---

This PR depends on (and should land after) the following PR:

• [Nexthop][NPI] "asic_config_v3" - Add tahan800bc platform and its variants #1242

The rack and test_fixture variants set platform_mapping_name to target sibling platform_mapping_v2 directories; the generator support for platform_mapping_name is introduced in #1242.