search: Refactor suggestions and add integration tests for them

Created by: camdencheek

Motivation: earlier today, a change I made that touched search suggestions caused a panic in frontend that took down cloud. I identified two things that would have helped prevent that outage. First was having integration tests that covered suggestions. Even a minimal test would have caught this panic. Second was refactoring the search suggestion code so that it relied less on interface{} so that static analysis would catch type errors.

Fixes #18621 (closed) Fixes #18622 (closed)

Overview of the changes:

• Make SearchSuggestionResolver an interface
• Create wrapper type for each type of suggestion that implements SearchSuggestionResolver
• Move core suggestion types into search_suggestions.go
• Convert consumers of newSearchSuggestionResolver to use the wrapper types directly
• Add an integration test

I apologize in advance for the large changes -- I struggled to split this one up.