APi to retrieve metadata version of instance

### Is your proposal related to a problem?

We're running heavy graphql codegen for our applications, the growth of our graph increase codegen workload significantly over time.

### Describe the solution you'd like

An API or a way to detect if the graph/metadata has changed so codegen will only create types or downloading schema when there are changes. I believe internally Hasura has that inside

### Describe alternatives you've considered

At the moment we hack to enable this data: add hasura metadata db itself into hasura, set a native query:

```sql
SELECT resource_version FROM hdb_catalog.hdb_metadata;
```

then run this graphql query:

```gql
{ hdb_catalog { metadata_version { resource_version } }}
```

Start by examining the existing metadata_version GraphQL query and the hdb_catalog.hdb_metadata resource_version value described in the issue. Determine how an API can expose metadata changes without requiring schema downloads, then verify that code generation can use the returned version to skip unchanged schemas.

Written by the indexing model from the issue text.