Add shortcut to get all component names from a coordinate object

Lately, I've had a few use cases where I need to grab all (6 in my case) of the position and velocity components off of a frame or SkyCoord instance. The code to do this is only a few lines, but many characters! So I'm wondering what you think about adding a convenience property that does something like:

```python
@property
def component_names(self):
names = list(self.get_representation_component_names().keys())

for key in self.data.differentials:
names += list(self.get_representation_component_names(key).keys())

return names
```

cc @mhvk @eteq

Start by reviewing the Frame and SkyCoord entry points, the existing get_representation_component_names() method, and the data.differentials access shown in the issue. Done means a convenience component_names property returns the position and differential component names for coordinate objects; no file or test is named, so those still need to be located.

Written by the indexing model from the issue text.