Support lua scripting to extend GraphQL server with a custom endpoint

[chipsenkbeil]

Leverage rlua as an optional feature in vimwiki-server to provide the entrypoint into a custom GraphQL call. Because I don't think we can dynamically create GraphQL schemas, we will instead provide a single type (query, mutation, and maybe subscription?):

{
    lua_query(key: "some identifier", input: "some input") {
        key
        output
    }
}

If dynamic schemas were possible as described in async-graphql/async-graphql#30, then lua could be used to make the schemas, although from what was described there it would be a high performance cost.