Validator for publishing rate

[jonasfj]

Proposal:

If you've published more than 5 versions over the past 30 days, then publishing validations throw an error.

This can be ignored with --skio-validation.

We can fetch versions from the API, which has a published date-time.

It's a simple way to limit publishing without making a hard limit. Also we can put in a message pointing to dart.dev/go/publishing-frequency. Then we can document guidelines for publishing rate, and document out limits.

[sigurdm]

Let's start by writing dart.dev/go/publishing-frequency. It will add value on its own

[sigurdm]

We should probably make this a warning only...