Public environment variables

### What problem are you trying to solve?

Some of my environment variables are not secret and it would be useful for me to see what values they have after I've set them in Deno Deploy. For example, say you had an environment variable for a rate limiter called MAX_REQUESTS_PER_MINUTE. Such an environment variable isn't necessarily secret and knowing the current value would be useful if you wanted to experiment to find a suitable level for your rate limiter.

### Describe the solution you'd like

GitHub has the concepts of Secrets and Variables for GitHub actions. The values of Secrets cannot be seen once they've been set, whereas Variables can be viewed and edited. I think a similar solution for Deno Deploy would be great!

### Describe alternatives you've considered

I've considered 2 alternatives:

1. I use deployctl within a GitHub action already. So, I could store my env vars in GitHub rather than Deno Deploy and pass them through to the deployment as part of my GitHub action. That way, I could use GitHub Secrets and Variables. One downside of this is that I'd most likely have to update my GitHub action each time I want to introduce a new env var.
2. I could store these types of env vars as values in my DB. However, that would mean that my service would depend on a DB connection in order to read its config, which I'm not that keen on.

### Documentation, Adoption, Migration Strategy

In terms of migration, I imagine that public env vars would be purely additive, so any existing env vars would remain as secret env vars. Users would then have the choice to use public env vars going forwards.

I imagine that public env vars could live in the same box as the existing (secret) env vars. Secret env vars would remain as they are, but public env vars would be readable and editable. Please see my very crude mock-up below 😅


No files, tests, or entry points are named in the issue. Start by reviewing how Deno Deploy currently manages secret environment variables and use the attached mock-up as product context; done means supporting readable and editable public variables alongside secrets while preserving existing secret variables.

Written by the indexing model from the issue text.