feat(create-plugin): generate RTK Query API clients in the experimental-app-sdk addition

[amalavet]

What this PR does / why we need it:

Extends the experimental-app-sdk addition so an app plugin also gets typed RTK Query clients for its kinds and custom routes, generated the same way core Grafana generates @grafana/api-clients:

src/app-sdk-manifest.json
  → grafana cli write-openapi          (grafana/grafana#132068, main)
  → .config/app-sdk/openapi/*.json     (gitignored)
  → grafana-api-clients generate       (grafana/grafana#132913, @grafana/api-clients bin)
  → src/api/generated/<version>/{baseAPI,index,endpoints.gen}.ts

Added to the scaffold, following the shape of generate-kinds.mjs:

• .config/app-sdk/generate-api-client.mjs — downloads the pinned Grafana release's grafana binary once into node_modules/.cache/grafana/<version>/ (GRAFANA_BIN overrides), runs write-openapi, then npx grafana-api-clients generate.
• npm scripts generate:api-client and generate (kinds + clients); existing scripts are never overwritten.
• dependencies @grafana/api-clients, @reduxjs/toolkit, react-redux.
• .gitignore entry for the intermediate OpenAPI documents.
• drift workflow now runs generate and checks src/api/generated too.
• .config/app-sdk/README.md, kinds/README.md, .config/AGENTS/app-sdk.md describe the hooks and store wiring.

Today every app plugin hand-rolls a getBackendSrv() wrapper per kind; this gives them the same generated hooks core's frontend uses (useList<Kind>Query, useCreate<Kind>Mutation, …), with cache invalidation, for free.

Which issue(s) this PR fixes:

Related to grafana/grafana-app-sdk#1493, grafana/grafana-app-platform-squad#111

Special notes for your reviewer:

• Depends on two unreleased pieces in grafana/grafana; both are usable today without waiting:
  • grafana cli write-openapi is on main only. The script pins nightly build 13.3.0-35040200895 (commit b8b4b49a, resolvable via the build id = Actions run id) and documents how to pick another. Switch to 13.3.0 once tagged.
  • The grafana-api-clients bin is API Clients: add grafana-api-clients generate CLI for app plugins grafana#132913. The script installs @grafana/api-clients@^13.3.0 on first run; until that is on npm, GRAFANA_PR=132913 npm run generate:api-client installs the tarball that PR's CI built (every grafana PR packs all @grafana/* into the buildPr artifact) using gh. The addition itself no longer adds @grafana/api-clients to package.json, so add experimental-app-sdk installs cleanly.
• Verified end to end on grafana/grafana-app-sdk-testing-plugin (kind + status + 3 custom routes): add experimental-app-sdk from this branch on a clean checkout, then GRAFANA_PR=132913 npm run generate:api-client — installed the PR build, downloaded Grafana, rendered v0alpha1 + v1alpha1, generated both clients; tsc reports 0 errors in src/api/generated.
• Idempotent like the rest of the addition: re-running skips existing files/scripts/gitignore entry.
• Steve: this branches from upstream main, not your fork (the addition isn't on stevesg/main). Happy to retarget.

_{PR involved assistance from anthropic:claude-fable-5-1}

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