fix: Configure gRPC keep alive as 120 sec

[surbhigarg92]

This PR modifies gRPC keepalive_time_ms from 30 sec to 120 sec as per the other languages (Refer Java )

This also removes the gRPC keepalive_timeout_ms configuration, the default configured for gRPC is 20 seconds

These settings can be over written by passing to spanner options

const spanner = new Spanner({ 
    'grpc.keepalive_time_ms': 30000,
  });