Add backup/fallback gRPC endpoint support

[jkao97]

Summary

• Adds BackupEndpoint struct for configuring backup gRPC endpoints with optional auth tokens
• Backup endpoints inherit the primary auth token if not specified
• Automatic failover during initial connection and reconnection attempts
• Preserves last_fetched_version across endpoint switches for data continuity

Usage

transaction_stream_config:
  indexer_grpc_data_service_address: "https://primary.aptoslabs.com:443"
  auth_token: "primary_token"
  request_name_header: "my-processor"

  backup_endpoints:
    - address: "https://backup1.aptoslabs.com:443"
      # Uses primary auth_token
    - address: "https://backup2.example.com:443"
      auth_token: "different_token"  # Different provider

Test plan

• Unit tests for config endpoint helpers (test_config_endpoint_helpers)
• Test failover to backup endpoint (test_failover_to_backup_endpoint)
• Test all endpoints exhausted error (test_all_endpoints_exhausted)
• Test backward compatibility with no backup endpoints (test_backward_compatibility_no_backup_endpoints)
• Existing timeout test still passes

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