Support generated server side fragments

It would be so great if Hasura supports some kind of server side fragments. Auto-generated or created by user through UI.
It allows the user to change set of fields in single place and do not duplicate field set in the code.

Example:
1) Fragment generated and stored on server side:
```
fragment server_side_fragment on my_entity {
id
name
valid_from
valid_till
version_id
}
```
2) Using server side fragment in all queries:

```
query getEntities() {
my_entity(id: "7") {
...server_side_fragment
}
}
```