Pass in type_of_union for decoding union type

[jliu-palantir]

Before this PR

After this PR

==COMMIT_MSG== Pass in type_of_union to the constructor when decoding a union type. Relevant PR to conjure-python: https://github.com/palantir/conjure-python/pull/681 ==COMMIT_MSG==

Possible downsides?

[changelog-app[bot]]

Generate changelog in changelog-dir>`changelog/@unreleased`</changelog-dir

Type

- [ ] Feature - [ ] Improvement - [x] Fix - [ ] Break - [ ] Deprecation - [ ] Manual task - [ ] Migration

Description

Pass in type_of_union to the constructor when decoding a union type. Relevant PR to conjure-python: https://github.com/palantir/conjure-python/pull/681 **Check the box to generate changelog(s)** - [x] Generate changelog entry

[johnhany97]

bump checks?

[svc-autorelease]

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