Add official OpenAPI YAML for Apollo OpenAPI

[tacklequestions]

Is your feature request related to a problem? Please describe.
There is no official, machine-readable API specification for Apollo’s OpenAPI endpoints. Integrators have to read code or scattered docs to discover endpoints and parameters, which leads to:

• duplicate client implementations and inconsistent behavior,
• harder onboarding and testing,
• no automatic SDK generation or contract validation.

Describe the solution you'd like
Provide and maintain an official OpenAPI 3.0 YAML for Apollo’s OpenAPI endpoints, and publish browsable docs.

Proposed plan:

• Add apollo-openapi.yaml (OpenAPI 3.0.x).
• Version the spec with releases; keep operationId stable for SDKs.
• Include request/response schemas, enums, and examples.
• (Optional) Provide openapi-generator scripts to generate Java/Go/TS SDK stubs.

Describe alternatives you've considered

• Postman collections or wiki pages (not type-safe, hard to keep in sync).
• Reading controller code (steep learning curve for external users).
• Community-maintained specs (can drift from upstream behavior).

Additional context
I have a draft spec in my fork/branch and can open a PR to iterate together:
tacklequestions/apollo:feat/openapi-yaml
It currently covers core resources/tags such as Apps, Configs, Env, Clusters, Namespace, Item, Release with endpoints like:

• POST /apps, GET /apps/{appId}, PUT /{appId}, DELETE /apps/{appId}
• GET /apps/{appId}/envclusters
• POST /apps/{appId}/envs/{env}/clusters
• POST /apps/{appId}/namespaces, etc.

Happy to align naming, tags, and folder structure with maintainers’ preferences and expand coverage in follow-ups.