Fix errors for unions with unit target membershape (#3547)

## Motivation and Context

Unions that have a [unit target member
shape](https://smithy.io/2.0/spec/model.html#unit-type) do not have an
associated data in the generated Rust enum.

Closes Issue:
[2546](https://github.com/smithy-lang/smithy-rs/issues/2546

)

## Description

On the **server** side, when the union has constrained members, the code
generated for the conversion from the `Unconstrained` type to the
`Constrained` type incorrectly assumed that each Rust enum would have
associated data.

```
rust-server-codegen/src/unconstrained.rs:31:129
  |
  |               crate::unconstrained::some_union_with_unit_unconstrained::SomeUnionWithUnitUnconstrained::Option1(unconstrained) => Self::Option1(
    |                                                                                                                                   -^^^^^^^^^^^^- help: consider using a semicolon here to finish the statement: `;`
    |  _________________________________________________________________________________________________________________________________|
  | |
  | |                 unconstrained
  | |             ),
  | |_____________- call expression requires function
    |
   ::: rust-server-codegen/src/model.rs:152:5
    |
    |       Option1,
    |       ------- `SomeUnionWithUnit::Option1` defined here
```

The marshaling code for event streams with unit target types incorrectly
assumed that the variant would have associated data.

```
rust-server-codegen/src/event_stream_serde.rs

impl ::aws_smithy_eventstream::frame::MarshallMessage for TestEventMarshaller {
        fn marshal() {
            let payload = match input {
                Self::Input::KeepAlive(inner) => {
```

On the **client** side, the `event_stream_serde` code incorrectly
assumes that a union member, which has the `@streaming` trait applied to
it, takes a `model::Unit` type.

```
rust-client-codegen/src/event_stream_serde.rs:
    |
    |                     crate::types::TestEvent::KeepAlive(crate::types::Unit::builder().build()),
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------------------------------
    |                     |
    |                     call expression requires function
    |
   ::: rust-client-codegen/src/types/_test_event.rs
    |
    |     KeepAlive,
    |     --------- `TestEvent::KeepAlive` defined here
```

## Testing

A unit test has been added that tests the following model:

```
            $version: "2"
            namespace com.example
            use aws.protocols#restJson1
            use smithy.framework#ValidationException
            
            @restJson1 @title("Test Service") 
            service TestService { 
                version: "0.1", 
                operations: [ 
                    TestOperation
                    TestSimpleUnionWithUnit
                ] 
            }
            
            @http(uri: "/testunit", method: "POST")
            operation TestSimpleUnionWithUnit {
                input := {
                    @required
                    request: SomeUnionWithUnit
                }
                output := {
                    result : SomeUnionWithUnit
                }
                errors: [
                    ValidationException
                ]
            }
            
            @length(min: 13)
            string StringRestricted
            
            union SomeUnionWithUnit {
                Option1: Unit
                Option2: StringRestricted
            }

            @http(uri: "/test", method: "POST")
            operation TestOperation {
                input := { payload: String }
                output := {
                    @httpPayload
                    events: TestEvent
                },
                errors: [ValidationException]
            }
            
            @streaming
            union TestEvent {
                KeepAlive: Unit,
                Response: TestResponseEvent,
            }
            
            structure TestResponseEvent { 
                data: String 
            }            

```

---------

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