fix: support self-hosted Elasticsearch via --host/--port in elasticcloud commands

[xiaofan-luan]

Summary

• Extend ElasticCloudConfig to accept either cloud_id or a self-hosted connection (scheme / host / port / user).
• Add --host / --port / --scheme / --user flags (and make --cloud-id optional) on all four elasticcloudhnsw* subcommands.
• Existing --cloud-id based usage is unchanged — cloud_id still takes precedence when both are supplied.

Motivation (#758)

Users running stock Elasticsearch 8.x (self-hosted, on-prem, or Tencent/Aliyun-hosted stock ES) had no working VectorDBBench entrypoint:

• elasticcloudhnsw* required --cloud-id — no way to point at a host/port.
• tencentelasticsearch accepts --host/--port but hardcodes index_options.type = "vsearch" (Tencent-specific), which stock ES rejects with Unknown vector index options type [vsearch] for field [vector].

With this change, the correct mapping (hnsw, int8_hnsw, int4_hnsw, bbq_hnsw) can finally be used against self-hosted ES clusters:

vectordbbench elasticcloudhnsw \
  --scheme http --host rw-xxxxx --port 9204 \
  --user elastic --password 'pw' \
  --case-type Performance768D1M --m 16 --ef-construction 200 --k 10

Test plan

• elasticcloudhnsw --cloud-id ... --password ... --dry-run — builds {"cloud_id": ..., "basic_auth": (...)} (backward compatible).
• elasticcloudhnsw --host rw-test --port 9204 --scheme http --password ... --dry-run — builds {"hosts": [{...}], "basic_auth": (...)}.
• elasticcloudhnsw --password ... --dry-run (neither flag) — rejected with ElasticCloudConfig requires either cloud_id or host to be set.
• ruff check vectordb_bench/backend/clients/elastic_cloud/ — passes.
• Manual run against a real self-hosted Elasticsearch 8.x cluster (requires reporter env).

Closes #758

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