Unable to pass FieldArg as parameter for resource .get()
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### Summary

Passing FieldArg as one of the parameters of `resouce.get({})` results in an error.

### Steps to reproduce

```typescript
return comments.get({
  post_id: $parentStep.get('id'),
  user_id: fieldArgs.get('user_id'),
});
```

### Expected results

The comment to be returned.

### Actual results

```
Error occurred during optimize; whilst processing PgSelect{1}<comments>[20] in dependents-first mode an error occurred: Error: Expected __TrackedValue➊[18] (1th dependency of PgSelect{1}<comments>[20]; step with id 2) to be a PgClassExpressionStep
```

### Additional context

I could work around this by instead doing
```typescript
const commentsSelect = comments.find({ post_id: $parentStep.get('id') });
const userIdPlaceholder = commentsSelect.placeholder(fieldArgs.get('user_id'));
commentsSelect.where(sql`user_id = ${userIdPlaceholder}`);
return commentsSelect.single();
```
but this feels clunky.

In grafast-vbeta.6 and postgraphile-vbeta.21 I had this workaround working
```typescript
return comments.get({
  post_id: $parentStep.get('id'),
  user_id: (fieldArgs.getRaw('user_id') as __TrakcedValueStep).optimize(),
});
```
but this is resulting in the same error in the latest versions.


### Possible Solution

<!--- Optionally suggest a fix or reason for the bug. -->
