Add support for custom index on hasMany / expose a list query on the secondary index created by hasMany

### Environment information

```plain text
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aws-amplify: 6.4.4
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```

### Description

In this version, hasMany will create a secondary index for internal queries but:
- the secondary index dose not expose a list query in AppSync and subsequently in the client.
- we can't assign a user-created secondary index to hasMany (as in v1)

I find this important for the case where we need to query the secondary index to find children by "parentId" given that I have the parent id.

Let's assume we have this:
```
const schema = a.schema({
Todo: a
.model({
content: a.string(),
description: a.string(),
comments: a.hasMany("Comment", "parentId"),
})
.authorization((allow) => [allow.guest()]),

Comment: a
.model({
content: a.string(),
parentId: a.string().required(),
todo: a.belongsTo("Todo", "parentId"),
})
.authorization((allow) => [allow.guest()]),
});
```

if i need to load the todo and the comments, i can build an efficient query with selectors:

```
const { data: todos, errors } = await client.models.Todo.list({
selectionSet: [
"id",
"content",
"description",
"comments.*",
],
});
```

but i can't have a proper way to get the Comments by todoId.

The only thing I found was **lazy loading the Comments** by passing the todo:

``` const { data: comments } = await todo.comments() ```

This will work in POCs and small projects, but **it uses a filter instead of secondary index** to get data so it will scan the table each time.

If we crete a manual secondary index, we double the index storage and we also count towards the max number of indexes per table.

If we can create a secondary index and declare it for usage in hasMany, all the above would be solved.