Expose certain calls by a specifiable name

Re: https://github.com/AllenNeuralDynamics/one-liner/pull/1#discussion_r2479545705

Currently, to call a remote function, we need to lookup the corresponding object instance and method name and then call the method. It would nice to abstract out the object and method name to a single call like this:

```python
server = RouterServer()

# assign a name (or look it up from a config)
server.register_call(obj_name="xyz_stage", method_name="get_positions", call_name="get_xyz_positions")
```

Then one or more clients can call with:

```python
client = RouterClient()
time, positions = client.call("get_xyz_positions")
```