Add namespace_path support for Projects Graphql API

What does this MR do and why?

Add namespace_path support for Projects Graphql API. This will filter the results to be projects within that namespace (group or user).

Query Plans

• Cold: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/40731/commands/125151
• Warm: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/40731/commands/125152

References

• Closes #541499 (closed)
• Parameter is needed for migrating the existing admin projects dashboard into Vue

Screenshots or screen recordings

How to set up and validate locally

1. Go to /-/graphql-explorer in your GDK instance.
2. Run the following query:

{
  projects(namespacePath: "<namespace-fullpath>") {
    nodes {
      id
      fullPath
      visibility
    }
    count
  }
}

MR acceptance checklist

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