v5 missing support for http batching

### Feature description

https://github.com/graphile/crystal/blob/main/grafast/grafserv/src/utils.ts#L298C27-L298C56
This check bails when an array of queries is sent to grafserv over http.

### Motivating example

HTTP Batching is often used with Apollo client users to reduce the amount of http requests from the clients to the server. See https://www.apollographql.com/docs/react/api/link/apollo-link-batch-http
Postgraphile v4 did support this, so in a way not having this in v5 is a breaking change from v4 that is not documented.

### Breaking changes

Shouldn't be any

### Supporting development

I [tick all that apply]:

- [ ] am interested in building this feature myself
- [x] am interested in collaborating on building this feature
- [x] am willing to help testing this feature before it's released
- [x] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile