[APPS] Add OAuth auth for Apps uploads

[sdkennedy2]

Summary

Depends on #393.

Adds Apps upload OAuth wiring on top of the shared core OAuth/request helpers:

• adds Apps-scoped auth selection via apps.authOverrides.method and DD_APPS_AUTH_METHOD / DATADOG_APPS_AUTH_METHOD
• defaults Apps uploads to API-key auth
• adds getAuthenticatedRequest and the internal DoAuthenticatedRequest type
• makes upload context require doAuthenticatedRequest
• uses a throw-if-called dry-run request sentinel so dry-run does not require auth but still catches accidental network execution
• updates Apps README/config/tests for upload OAuth behavior

Validation

• yarn workspace @dd/tests test:unit packages/plugins/apps/src/auth.test.ts packages/plugins/apps/src/upload.test.ts packages/plugins/apps/src/index.test.ts packages/plugins/apps/src/validate.test.ts
• yarn cli integrity
• git diff --check

[sdkennedy2]

Closing this because the Apps upload OAuth wiring has been folded back into #393 after collapsing the stack. #421 remains the separate optional-dependency integrity PR.