Instance-wide searches with Zoekt

For now we've only implemented group and project scoped searches with Zoekt. We considered these the priority for GitLab.com as we won't be indexing the entire GitLab.com instance any time soon.

For self-managed customers they may wish to index their whole instance and allow instance wide searches using Zoekt. It should be pretty straightforward to support this the same way we do for Elasticsearch.

Based on https://github.com/sourcegraph/zoekt/pull/521#discussion_r1090647645 I learnt there is already an existing mechanism for storing a repo is public. Like zoekt.repoid this is stored in the git config (and index data) and can be queried as part of the API. This would help with making global searches work as we can't pass all ids of all public repos. We'd still need to update the zoekt-dynamic-indexserver to be passed this boolean for whether a project is public but it should be a small addition.