Write and publish environment TOMLs for `spin up`

[itowlson]

In order to start making use of the target environments feature, we need environment TOMLs (that is, trigger type-to-compatible worlds mappings) to be in an OCI registry. The default location is ghcr.io/spinframework/environments.

We should decide/define/create:

• Where to maintain the TOMLs that are part of CNCF definitions (bearing in mind there will also be SpinKube: e.g. spin up in the Spin/SpinKube in the SK repo, or both in a separate repo, or...)
• How to handle third-party TOMLs (PRed into the Spin org and pushed to the default registry, or separate and pushed to owner's registry, or some intermediate which I dunno what it would look like)
• Processes for updating the TOMLs with new Spin releases (particularly around timing once we start offering environments in templates)
• Automation around pushing new TOMLs to OCI