Use entrypoints for Data Plugins

[hhslepicka]

What's the problem this feature will solve?
Using entrypoints for data plugins will make so that PyDM can auto discovery data plugins available and provided by other packages.
It will also allow us to move the data plugins outside of the PyDM core package which will allow for a more dynamic and clean development of data plugins.

Describe the solution you'd like
Add a search to entrypoints for data plugins.

Additional context
entrypoints package: https://pypi.org/project/entrypoints/