feat: add automatic Vault secret loading for local E2E testing

[subhashkhileri]

JIRA: https://redhat.atlassian.net/browse/RHIDP-13050

Summary

• Adds loadLocalVaultSecrets() in src/utils/vault.ts — fetches secrets from
  Vault during globalSetup when VAULT=1 or VAULT=true
• Handles OIDC login automatically, fetches global + per-workspace secrets
  concurrently
• Only exports keys starting with VAULT_, skips secretsync/ metadata keys
• Logs key names only (never values), shows Slack channel #rhdh-e2e-tests on
  permission denied
• Configurable via VAULT_ADDR and VAULT_BASE_PATH env vars

Usage

# From workspace
yarn test:vault
# or
VAULT=1 yarn test

# From repo root
VAULT=1 ./run-e2e.sh -w argocd