Add support for many nested objects in SaveOperations

Original Issue: #43

I'm writing a new issue because the original one was created a long time ago and makes mention of a lot of things that no longer exist, though the sentiment is still the same.

We need a way to support creating and updating many nested records for a parent object.

Given a JSON payload that looks like this:

```json
{
"menu": {
"title": "Main Menu",
"active": true,
}
"items": [
{
"title": "First item",
"active": true,
"position": 0
},
{
"title": "Second item",
"active": false,
"position": 1
}
]
}
```

I should be able to use these params to create a new `menu` object as well as 2 new `item` objects. This must be able to support the ability to decide which of the nested keys are permitted and which are not similar to using `permit_columns` which helps to prevent updating unpermitted columns.

This is a rough idea of the intent:

```crystal
class SaveMenu < Menu::SaveOperation
permit_columns title, active

has_many items : SaveItem
end

class SaveItem < Item::SaveOperation
permit_columns title, active, position
end

# `params` here represents the above json structure
SaveMenu.create!(params)
```

Under the hood, we can use `params.many_nested(:items)` here. Ideally this should also update them in a single query. Each nested record could run through the operation to check all validations and run any data transformations that need to happen, then once all operations are `valid?` and have no `errors`, then it submits the single query.

Last thing to consider is if an `Item` requires a `menu_id`, then the `items` can't even begin to save until the `menu` has already been saved and created. This whole thing will need to be inside of a transaction so if the `items` fail to save, the transaction can be rolledback to remove the `menu`.

Start by reading the existing SaveOperations API and the permit_columns behavior, then trace how nested params are currently parsed. Done should include creating and updating many nested records with permitted fields, validation before persistence, and transaction rollback when a nested save fails.

Written by the indexing model from the issue text.