Add backend support for retrieving database schema results

Stan Hurequested to merge
sh-add-db-schema-diagnostics-page-backend into master

What does this MR do and why?

This is split out from !199796 (merged) to reduce the review overhead.

References

Relates to #467544.

How to set up and validate locally

Probably easiest way is to play with the frontend changes in !199796 (merged) for now.

MR acceptance checklist

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Edited by Stan Hu

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