Provide an option like externalGraphqlRoute for the CLI usage
### Feature description

Provide an option like externalGraphqlRoute for the client usage.

### Motivating example

Curently I cannot use the client behind a reverse proxy and serve graphql AND graphiql on a suburl.

I am trying to start postgraphile with pm2 to kame a persistent API endpoint in the most simple way.

graphql can be served on a suburl without any problems but graphql searches for graphql under root and therefore no endpoint is availible).

We have similar cases in our working group and externalGraphqlRoute provides a simple solution to this problem.

But there is no client option for this now.

### Supporting development

I [tick all that apply]:

<!-- To tick a checkbox, change it from `[ ]` to `[x]` -->

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [x] am willing to help testing this feature before it's released
- [ ] 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
