search: alerts expose Kind field

Created by: rvantonder

This exposes a Kind field for alerts so clients have more descriptive ways of identifying kinds of alerts. The main motivation is to use a new client-side component for rendering suggested queries, that's different from an older component, and we want to distinguish between the two.

It could also be used indicate an error level or other information, but the contract here is loose: I don't see a need to use enums to enumerate kind here, because I don't know what a sensible enumeration looks like. It's useful to just have a string label and acknowledge that the value of kind is optional, and a loose contract until this becomes less foggy.

Test plan

Intended to be semantics-preserving and unused. backend-integration added in case any code doesn't like this being undefined/empty.