Add support for grayscale emitter

Currently, we can only use the RGB emitter with a specified color, e.g.
```
layer = {
"source": source,
"tab": "rendering",
"blend": "additive",
"shader": {
"color": "red",
"emitter": "RGB",
"vec": "vec3",
},
"shaderControls": { # Optional
"normalized": {"range": [vmin, vmax]}
},
}
```

If you try to use the grayscale emitter like so, it causes an exception
```
layer = {
"source": source,
"tab": "rendering",
"blend": "additive",
"shader": {
"emitter": "grayscale",
},
"shaderControls": { # Optional
"normalized": {"range": [vmin, vmax]}
},
}
```

```
Traceback (most recent call last):
File "/home/svc_aind_upload/cameron.arshadi/repos/aind-data-transfer/scripts/create_neuroglancer_links.py", line 180, in
main()
File "/home/svc_aind_upload/cameron.arshadi/repos/aind-data-transfer/scripts/create_neuroglancer_links.py", line 176, in main
write_json_from_zarr(args.input, args.output, args.vmin, args.vmax)
File "/home/svc_aind_upload/cameron.arshadi/repos/aind-data-transfer/scripts/create_neuroglancer_links.py", line 163, in write_json_from_zarr
neuroglancer_link = NgState(
File "/home/svc_aind_upload/anaconda3/envs/adt-ca/lib/python3.10/site-packages/ng_link/ng_state.py", line 61, in __init__
self.initialize_attributes(self.input_config)
File "/home/svc_aind_upload/anaconda3/envs/adt-ca/lib/python3.10/site-packages/ng_link/ng_state.py", line 275, in initialize_attributes
self.layers = input_config["layers"]
File "/home/svc_aind_upload/anaconda3/envs/adt-ca/lib/python3.10/site-packages/ng_link/ng_state.py", line 213, in layers
NgLayer(
File "/home/svc_aind_upload/anaconda3/envs/adt-ca/lib/python3.10/site-packages/ng_link/ng_layer.py", line 101, in __init__
self.update_state(image_config)
File "/home/svc_aind_upload/anaconda3/envs/adt-ca/lib/python3.10/site-packages/ng_link/ng_layer.py", line 260, in update_state
self.shader = self.__create_shader(value)
File "/home/svc_aind_upload/anaconda3/envs/adt-ca/lib/python3.10/site-packages/ng_link/ng_layer.py", line 296, in __create_shader
color = shader_config["color"]
KeyError: 'color'
```

It would be useful to add an option to use the grayscale emitter, especially for single channel datasets as the RGB emitter does not look as good in neuroglancer