Allow providing SSL CA certificates to graphile-migrate CLI

### Feature description

I am looking for a way to provide a path to additional SLL certificate(s) for the database connection strings.

```sh
GRAPHILE_SSL_CA_CERT=./some-cert-path graphile-migrate migrate
```

Or via the config file, passing the options along to the underlying pg-pool:

```json
{
"pgopts": { "ssl": { "ca": "path-to-cert" } }
}
```

### Motivating example

I am attempting to use graphile-migrate (2.0.0-rc.2) to migrate a DigitalOcean managed database, which as [previously recognized](https://github.com/graphile/migrate/issues/228) uses self-signed certificates.

### Supporting development

I [tick all that apply]:

- [ ] 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