Upgrade Smithy to 1.40 and ignore __type in unions (#3081)

## Motivation and Context
> JSON object. A union is serialized identically as a ``structure``
        shape, but only a single member can be set to a non-null value.
Deserializers MUST ignore an unrecognized ``__type`` member if present.


[source](https://github.com/smithy-lang/smithy/blob/main/docs/source-2.0/aws/protocols/aws-json.rst.template#L133-L135)

## Description
- upgrade to smithy 1.40
- unignore protocol tests
- fix JSON deserializers
## Testing
- protocol tests + extra unit tests

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