feat(serverless): Allow passing auth headers to webhooks

[khushhhhh]

Problem

Resolves #413.
Currently, the serverless worker hardcodes the headers for webhook transmissions (Content-Type and charset). When users need to secure their webhook endpoints (e.g., using a bearer token or API key to prevent unauthorized payloads), there is no built-in way to pass an Authorization header back to the configured endpoint.

Reproduction

1. Configure a serverless endpoint with RUNPOD_WEBHOOK_POST_OUTPUT pointing to an authenticated URL (e.g., expecting a Bearer token).
2. The payload fails to deliver with a 401/403 status because the worker strips or ignores any authentication credentials.

Fix

Updated _transmit inside runpod/serverless/modules/rp_http.py to dynamically check for RUNPOD_WEBHOOK_AUTHORIZATION or RUNPOD_WEBHOOK_HEADER_AUTH environment variables. If present, it injects the Authorization header into the outgoing POST request payload. This allows secure endpoints without breaking backward compatibility for unauthenticated endpoints.

Testing

• Verified locally by setting RUNPOD_WEBHOOK_AUTHORIZATION="Bearer token" and intercepting the webhook output.
• All existing tests pass.