Fix directly constrained `List` shape with indirectly constrained aggregate...

Fix directly constrained `List` shape with indirectly constrained aggregate type member shape (#3894)

This PR fixes a bug in the code generation for a directly constrained
List shape that has an indirectly constrained aggregate type as a member
shape.

For example, in the following model:

```smithy
@http(uri: "/sample", method: "POST")
operation SampleOp {
  input := {
      a: ItemList
  }
  errors: [ValidationException]
}
@length(min: 1 max: 100)
list ItemList {
  member: Item
}
map Item {
  key: ItemName
  value: ItemDescription
}
@length(min: 0 max: 65535)
string ItemName
string ItemDescription
```
A `TryFrom` implementation is generated that converts from
`ItemListUnconstrained` to `ItemList` by converting each member of the
inner list of `ItemUnconstrained` to `ItemConstrained` and then
converting it to `Vec<Vec<ItemName>>`:

```rust
    impl std::convert::TryFrom<ItemListUnconstrained> for crate::model::ItemList {
        type Error = crate::model::item_list::ConstraintViolation;
        fn try_from(value: ItemListUnconstrained) -> Result<Self, Self::Error> {
            let res: Result<
                ::std::vec::Vec<
                    ::std::collections::HashMap<crate::model::ItemName, ::std::string::String>,
                >,
                (usize, crate::model::item::ConstraintViolation),
            > = value
                .0
                .into_iter()
                .enumerate()
                .map(|(idx, inner)| {
                    inner.try_into()
                        .map(|ic: crate::constrained::item_constrained::ItemConstrained| ic.into())
                        .map_err(|inner_violation| (idx, inner_violation))
                })
                .collect();
            let inner =
                res.map_err(|(idx, inner_violation)| Self::Error::Member(idx, inner_violation))?;
            Self::try_from(inner)
        }
    }

```

Partially fixes issue: #3885 for publicly constrained shapes only.

---------

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>