SOAR-0009 - Typesafe multipart with streaming (#369)

SOAR-0009 - Typesafe multipart with streaming

### Motivation

Ready for a review, see rendered at: https://github.com/czechboy0/swift-openapi-generator/blob/hd-soar-0009/Sources/swift-openapi-generator/Documentation.docc/Proposals/SOAR-0009.md

### Modifications

N/A

### Result

N/A

### Test Plan

N/A


Reviewed by: 

Builds:
     ✔︎ pull request validation (5.10) - Build finished. 
     ✔︎ pull request validation (5.8) - Build finished. 
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     ✔︎ pull request validation (compatibility test) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
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     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#369

Author: czechboy0

Sources/swift-openapi-generator/Documentation.docc/Proposals/Proposals.md

@@ -50,3 +50,4 @@ If you have any questions, tag [Honza Dvorsky](https://github.com/czechboy0) or
 - <doc:SOAR-0005>
 - <doc:SOAR-0007>
 - <doc:SOAR-0008>
+- <doc:SOAR-0009>