insights: add initial DB schema

Created by: slimsag

This change adds the initial code insights DB schema. It is generally well-thought-out and I did a fair amount of analysis on whether or not to use more involved TimescaleDB features like compression instead of / in addition to storing repo names as IDs, for example.

I am sure we will change the schema as we learn more, but this generally seems sufficient/appropriate for now.

Helps #17218

Signed-off-by: Stephen Gutekanst [email protected]