[Plugin] Allow users to discover, choose and install community plugins

[abhinavsingh]

Is your feature request related to a problem? Please describe.
Plugins are currently packaged or must be downloaded/developed locally. There is no system to explore or discover proxy.py plugins. There are some on GitHub at this point in time.

Describe the solution you'd like
Allow users to discover, choose and install plugins.

Additional context
Following options come to mind (if you have a better suggestion, kindly suggest):

1. We could have a separate repository for plugin only. This repo can act as source of all community shared plugins
2. Dynamically explore plugins on GitHub using static checker e.g. all implementations of proxy.py base plugin classes
3. Each plugin as a separate repository, installable via pip install proxy.py[plugin-name]. We'll still need a registry to map plugin-name to git repo. This registry of plugin names to repo can exist in proxy.py repo itself. Or discoverable via a gist/separate repo
4. Any other option?