tag schema versions

Today we always only serve the latest schema version artifacts from the CDN from the following URL:

```
https://cdn.graphql-hive.com/artifacts/v1//supergraph
https://cdn.graphql-hive.com/artifacts/v1//sdl
https://cdn.graphql-hive.com/artifacts/v1//services
```

We can introduce tagging to give more control over which schema version is served to a specific router/gateway instance

A tag would be applied via the Hive CLI (and/or an API call and/or Dashboard UI).
The state of the tag would then be stored on our end in the database, so we can show which tag is the latest schema version and also quickly retrieve the schema version for a specific tag.

On the CDN side, the artifacts would then be accessible via the a URL similar to this:

```
https://cdn.graphql-hive.com/artifacts/v1//tags//supergraph
https://cdn.graphql-hive.com/artifacts/v1//tags//sdl
https://cdn.graphql-hive.com/artifacts/v1//tags//services
```

Related issues:
- https://github.com/graphql-hive/console/issues/7222