fix: support allocator down for vacating cluster resources

[Z3r0Sum]

Description

• The actual vacate request that was being used on cluster resources did not propagate down the flag being set for "VacateParams"
• This leads to a request that ultimately does not include the query param when performing the vacate

Related Issues

Motivation and Context

Unable to pass --allocator-down true and have it be respected from ecctl's perspective.

How Has This Been Tested?

UTs and will validate with a local version of ecctl (TODO)

Types of Changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (improves code quality but has no user-facing effect)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation

Readiness Checklist

• [x] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed (existing test failing on master)

[Z3r0Sum]

I was able to validate this locally with a run of ecctl pointing at this change:

POST /api/v1/regions/aws-eu-west-1/platform/infrastructure/allocators/i-0c1d25c3a0cb8659e/clusters/elasticsearch/_move?allocator_down=true HTTP/1.1

{"apm_clusters":null,"appsearch_clusters":null,"elasticsearch_clusters":[{"cluster_ids":["558c087999324a3d99656ebfcfed34fe"],"plan_override":{"plan_configuration":{"calm_wait_time":5,"extended_maintenance":false,"max_snapshot_age":300,"max_snapshot_attempts":3,"move_allocators":[{"allocator_down":true,"from":"i-0c1d25c3a0cb8659e","to":null}],"move_instances":null,"move_only":true,"override_failsafe":false,"preferred_allocators":[],"reallocate_instances":false,"skip_data_migration":true,"skip_post_upgrade_steps":false,"skip_snapshot":true,"skip_snapshot_post_major_upgrade":false,"skip_upgrade_checker":false,"timeout":8192},"strategy":{"grow_and_shrink":{}}}}],"enterprise_search_clusters":null,"kibana_clusters":null}