dependencies: add GraphQL resolver to list lockfile indexes

Created by: mrnugget

This is just a tiny resolver and a lot of boilerplate. I'll need to extend the LockfileIndex resolver to have more attributes, etc.

But I want to get this in because there's so much boilerplate and high chance of yak-shaving around how to hook up services, resolvers, etc.

What's in here is a mixture of the executors resolvers and the patterns we have in other resolvers, such as the codeintel or batches resolvers.

Test plan

• Existing and new tests added in this PR
• Manual testing in API console