[Feature]: Google Slides piece - service account auth (CustomAuth)

Problem

Google Slides is OAuth2-only. packages/pieces/community/google-slides/src/lib/auth.ts exports a single PieceAuth.OAuth2, so the connection dialog offers only Client ID / Client Secret.

google-docs, google-sheets, google-drive, google-calendar and google-forms all ship a second "Service Account (Advanced)" PieceAuth.CustomAuth. Slides is the gap.

Ref: Pylon 6011

Proposed

Port the google-docs pattern (PR 11635) to google-slides:

• auth.ts exports [OAuth2, CustomAuth]; CustomAuth props: serviceAccount (JSON key, LongText, required) + userEmail (optional, domain-wide delegation), with validate.
• createGoogleClient / getAccessToken branching on AppConnectionType.CUSTOM_AUTH via JWT from google-auth-library (already a dependency of the piece).
• Route the 3 actions and createCustomApiCallAction authMapping through getAccessToken.

OAuth2 stays default; existing connections unaffected.