topsql_v2: add TiKV collection toggle

[zeminzhou]

Summary

• add enable_tikv_topsql to topsql_v2 config
• keep current behavior by default with enable_tikv_topsql = true
• skip TiKV TopSQL subscriptions when the config is set to false
• document the new config in topsql_v2 architecture doc

Testing

• cargo test filter_topsql_components --lib

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