VoltageLevel.getConnectables() should be optimized

### Describe the current behavior

VoltageLevel.getConnectables() currently does 15 individual requests sequentially.

### Describe the expected behavior

 We could at least do them in parallel or better yet do a single request and implement on the server (like network.getIdentifiable())

### Describe the motivation

performance and less resource usage

### Extra Information

_No response_