Construct GITLAB_THROTTLE_USER_ALLOWLIST from a more detailed list

The env var must be just a list of user ids, but we want to also note the username (for humans) and link to an issue where any remediation is being tracked, because these sort of bypasses are expected to be temporary while we work on fixes (better APIs, tiered rate-limits, etc).

This change causes the cookbook to construct the environment variable value from a more detailed data structure. In practice, the name and issue fields are arbitrary and not specifically required, just exemplars and the intended usage pattern. The fact it is a hash will ensure operators provide a value, so this should encourage at least a comment. I do not think it wise to require a specific value (hash with known fields) because the only way to enforce that is by chef failing a verification step at run time when rendering gitlab.rb, and our goal here is not worth the problems that might cause.