[S7] Add OIDC auth to GitHub Action

[krakenhavoc]

Description

Update the GitHub Action to support OIDC authentication as an alternative to API keys.

Flow

1. Check if api-key input is provided
2. If yes, use API key auth (existing behavior)
3. If no, check for ACTIONS_ID_TOKEN_REQUEST_URL env var
4. Request OIDC token from GitHub with audience: https://api.krakenkey.io
5. Exchange OIDC token for KrakenKey access token via /v1/auth/github-oidc
6. Use KrakenKey token for all subsequent API calls

Requirements

• Workflow needs permissions: id-token: write
• Clear error message if neither API key nor OIDC is available
• Document OIDC setup in README

Acceptance Criteria

• OIDC auth works when no API key provided
• API key auth still works (backward compatible)
• Clear error if neither auth method is available
• README updated with OIDC examples