Ease of access / development for open Source contributors

[alexandreLamarre]

It can be hard to know where to get started if you want to add to Opni despite having developer documentation.

I propose two things

• an Opni dev container in .devcontainer folder in root for bundling all the opni dev / useful tools for engineering
• Related to https://github.com/rancher/opni/discussions/1008 as we narrow down plugin formats, a form of plugin template generation like :

mage -v generatePlugin --name "example" --gateway-enabled --agent-enabled

Also helps with on-boarding.