Add client-support for RPC v2 CBOR (#3767)

## Motivation and Context
Follow-up on https://github.com/smithy-lang/smithy-rs/pull/2544 to add
client-side support for the protocol

## Description
The client implementation mainly focuses on a sub-section
[Requests](https://smithy.io/2.0/additional-specs/protocols/smithy-rpc-v2.html#requests)
in the spec. To that end, this PR addresses `TODO` for the client to
fill in the blanks and includes additional adjustments/refactoring to
pass client protocol tests.

## Testing
- Existing tests in CI
- Upstream protocol test `rpcv2Cbor`
- Our handwritten protocol test `rpcv2Cbor-extras.smithy`

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