Auto-generated bulk insert mutations
### Feature description

The PostGraphile documentation [mentions bulk inserts](https://www.graphile.org/postgraphile/custom-mutations/#bulk-insert-example), but the example there is not super helpful for implementing a custom mutation for a rather common (I'd imagine) use case for inserting multiple records of the same type in one query.

It seems there are [other (commercial) products](https://hasura.io/blog/bulk-insert-mutations-with-graphql-e22727ce5b12/) who expose bulk inserts automatically and I'm wondering if there's a reason behind PostGraphile not making these available in the default mutations?

### Motivating example

I'm not a Postgres-expert, so it took me good bunch of research and learning about array and composite types and functions to finally come up with a solution that creates an endpoint which takes an array of table row records and bulk-inserts them:

```sql
-- header
CREATE TABLE "header" (
  "mid"    INTEGER,
  "type"   VARCHAR,
  "contents"  TEXT,
  FOREIGN KEY("mid") REFERENCES "email"("mid")
);
-- allow bulk-creating header rows
CREATE FUNCTION public.create_headers(create_multiple public.header[])
RETURNS SETOF public.header
AS $$
  INSERT INTO public.header
    SELECT (UNNEST(create_multiple)).*
    RETURNING *;
$$ LANGUAGE sql STRICT VOLATILE;
```

At the very least it might be useful to list (or, a simpler version, if one exists, this is the best I could come up with) in the docs, but having this as a built-in generated option seems like a useful thing to have, too.

### Breaking changes

Implementing this would be an additional exposed default mutation, I don't see that resulting in breaking changes.

### Supporting development

I [tick all that apply]:

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

- [ ] 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
- [ ] 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
