InfluxDB 3 Cloud: add OpenAPI spec for the initial (PR #7338) endpoint set

[jstirnaman]

Summary

Add an OpenAPI spec (and generated API reference) for InfluxDB 3 Cloud, scoped to only the endpoints that back the features included in #7338:

• Write: /api/v3/write_lp (and v1/v2 write-compatibility endpoints if in scope)
• Query: /api/v3/query_sql, /api/v3/query_influxql
• Databases: /api/v3/configure/database (create, list, delete)
• Tables: /api/v3/configure/table (create, delete)
• Tokens: admin token and database (resource) token endpoints under /api/v3/configure/token

Why

The Cloud CLI reference added in #7511 is deliberately restricted to the #7338 feature set (write, query, databases, tables, named admin tokens, database tokens). The API reference should match that surface exactly — no Processing Engine, caches, file index, system, or operator endpoints — so CLI and API docs stay consistent for the initial release.

Scope / notes

• Exclude endpoints for features not in this release (Processing Engine/plugins/triggers, caches, file index, nodes/license, etc.).
• Mirror the CLI split: reuse the Enterprise/Core spec where identical; keep Cloud-specific (hosted) differences separate.
• Related: Added initial cloud files #7338 (initial Cloud docs), docs(influxdb3-cloud): add influxdb3 CLI reference for Cloud #7511 (Cloud CLI reference).