[meta] Formalize runtime feature tracking process

[lann]

As the Spin runtime gains more downstream consumers I think we need to do a better job of tracking how new features get rolled out across downstream embedders (e.g. spin up, SpinKube), documentation, tooling, etc.

I would propose that every PR that introduces a new runtime feature must include answers (either directly or via linked tracking issue) to a few questions about how the feature will be rolled out.

Some example questions:

• Is this feature "expected" to be implemented by any/some/all embeddings apart from spin up?
  • What will be the UX of deploying (or trying to deploy) an app to an embedding that does not support this feature?
  • What is the expected level of difficulty for embeddings to implement this feature? e.g. should it "just work" with a Spin upgrade / trivial code changes?
• What documentation changes should be made to support this feature?

Additionally, if the answers to any of these questions indicate further work, a tracking issue must be created for the feature, to be resolved before the next Spin release including that feature.