Dual licensing - move all code in this repo to GPL after 3 years.

[tonyhb]

We're a small group trying to make queues better for people. We want people to be able to:

• Self host Inngest, with no fees, cost, or extra requirements.
• Improve Inngest and write their own code for integrating with their systems, if required
• Contribute via bug reports (or code, if desired)

We're using the SSPL to ensure that people can self-host for free, use us locally, use us in CI, etc. — while ensuring that larger companies can't take and resell us immediately.

That said, we also respect true open source. We should dual-license all code after a period of time so that it's free as recognized by OS foundations.