Add support for `post.image(size: SMALL)`-style singular relation fields based on unique constraints

### Feature description

It would be awesome if PostGraphile supported **parameterized singular relation fields** — basically, being able to fetch a **single related row by a unique key** directly on a parent type, without having to filter through a connection or write an extendSchema plugin by hand.

This is a really common schema pattern: you have a child table with a **unique constraint that involves the parent foreign key**, and you want to fetch exactly one related row using that constraint. PostGraphile already has everything it needs to figure this out from the database schema.

The idea is that PostGraphile could automatically add a singular field to the parent type with arguments matching the extra unique key columns. Under the hood it could batch these efficiently (e.g. `WHERE (parent_id, key) IN (...)`), so there’s no N+1 problem — just clean, expressive GraphQL.

### Motivating example

This comes up all the time in real schemas.
For example, imagine you want to fetch a single image of a given size for each post:

```sql
create table post (
id serial primary key,
headline text,
body text
);

create table post_image (
post_id integer references post,
size text check (size in ('small','big')) not null,
url text,
primary key (post_id, size)
);
```

**Desired GraphQL query:**

```graphql
query {
posts(first: 10) {
nodes {
id
image(size: small) {
url
}
}
}
}
```

This is clean and exactly matches how the data is structured in the DB — (_post_id, size_) uniquely identifies one row.

Another common case is **many-to-many or lookup by unique child key:**

```sql

create table author (
id serial primary key,
name text
);

create table post_author (
post_id integer references post,
author_id integer references author,
primary key (post_id, author_id)
);
```

**Desired GraphQL**

```graphql
post(id: 1) {
author(id: 42) {
name
}
}
```

And one more example — **user settings**:

```sql
create table user_setting (
user_id integer references app_user,
key text not null,
value text,
unique (user_id, key)
);
```

**Desired GraphQL**

```graphql
query {
currentUser {
setting(key: "dark_mode") {
value
}
}
}
```