Build API for exposing constants

Spotted in: https://github.com/elastic/elastic-package/pull/331

We need to expose constants like categories, releases, etc. via dedicated Go API, so it can be accessed in elastic-package. Otherwise we have to duplicate these values in multiple places.

I believe we should apply the same mechanism for regexps, for example - Github owner.

Start by reviewing the context in elastic/elastic-package#331 and locating where categories, releases, and regexps such as the GitHub owner are currently duplicated. Clarify the intended public Go API and its complete constant set before implementing it. Done means elastic-package can consume these values without maintaining duplicates.

Written by the indexing model from the issue text.