This adds a `message` column to the `alert_rule_version` table. This follows the
pattern established for dashboards as closely as possible. A new type is
introduced internally for passing the new `message` field around in a type-safe
manner, but doing the same for the API types becomes very messy. In that case, a
new field is added with omitempty.
Note this PR is only:
- The column addition
- The "read" path; API for listing versions
Subsequent PRs will add code to actually set the message when updating rules.
cc6e037093