Add sample and testing plugins

[sebinside]

There are several sample plugins to add to the public-plugins repo:

• A all-optional testing plugin to test several connectors, developed by @joblo2213

• A simple placeholder plugin for testing purposes

• Existing plugins should be removed

[sebinside]

Still missing:

• A simple placeholder plugin for testing purposes
• Existing plugins should be removed

But let me just finish this.

[sebinside]

Added in codeoverflow-org/chatoverflow-plugins@ae8ca84