[stacked] Box streaming request bodies

[sfackler]

Before this PR

Streaming request bodies were passed by reference at the Client level, while they were passed by value in the generated Conjure code. This complicated codegen and caused inconsistencies in behavior between conjure-codegen and conjure-macros.

After this PR

==COMMIT_MSG==
Streaming request bodies are now passed by value to the client.
==COMMIT_MSG==

We could go further and implement a bigger change to handle palantir/conjure-rust-runtime#146. However, this would complicate the interface even more and be only possible in the async client. Given that we don't have any use cases for full-duplex communications via Conjure clients it doesn't seem worth it.

Closes #259

[changelog-app]

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• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

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How are new versions calculated?

• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Streaming request bodies are now passed by value to the client.

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