Allow to override `pg` library

I'd like to use https://www.npmjs.com/package/pg-range with my `fastify-postgres`.

By default, `pg-range` is patching the `pg` in order to work.
```
var pg = require("pg");
require("pg-range").install(pg);
```

Currently, I can't override the `pg` (at least properly) used by `fastify-postgres`

Could we make an option for overriding it ?