Bump smithy version to 1.52 (#3887)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Bumping smithy to 1.52 for an `httpChecksum` trait bugfix

A few fixes had to be made to protocol tests for this release:
* Update document to initialize as empty `{}` instead of null
* Update our header serialization to serialize present but empty headers
to `""` (Note that this change does not apply to server header
serialization)
* Add `accept: application/cbor` header to add rpcV2Cbor requests
* Skip two broken tests. These tests were fixed in
https://github.com/smithy-lang/smithy/pull/2423 and that change was
included in the 1.52 release notes, but the actual change did not make
it into the build artifact. The changes should make it into the 1.52.1
release in the next few days:
https://github.com/smithy-lang/smithy/pull/2428

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