Add GraphQL query for shared projects

What does this MR do and why?

Related #562257 (closed)

Needed for Setup "Shared projects" tab (#554630 - closed). We are refactoring the groups overview tabs to shared Vue components. Currently we are using a REST API to get shared projects but it doesn't support all the attributes we need and the shared Vue components work better with GraphQL. This MR adds a GraphQL query for shared projects.

Implementation reasoning

At first I was having SharedProjectsResolver extend ProjectsResolver but there are a number of arguments that don't work with the finder and maybe don't make sense for this query such as membership and personal. This MR was also getting large because I then had to have tests for every single argument that was supported.

I think looked into creating a Resolvers::Projects::BaseResolver with only the shared logic in ProjectsResolver but this seemed like we were spreading logic across multiple files but it was confusing.

I decided it made more sense to start small and only support the arguments that we needed with a bit more duplication.

Screenshots or screen recordings

How to set up and validate locally

1. Go to a project -> Manage -> Members
2. Click Invite a group
3. Choose a group and invite
4. Make note of the group's path
5. Go to /-/graphql-explorer
6. Run the following query

{
  group(fullPath: "gitlab-org") {
    sharedProjects {
      nodes {
        id
        nameWithNamespace
      }
      count
    }
  }
}

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