[Proposal] SOAR-0004: Streaming request and response bodies (#254)

### Motivation

See the proposal at
https://github.com/czechboy0/swift-openapi-generator/blob/hd-soar-0004/Sources/swift-openapi-generator/Documentation.docc/Proposals/SOAR-0004.md.

### Modifications

N/A

### Result

N/A

### Test Plan

N/A

Author: czechboy0

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

@@ -46,3 +46,4 @@ If you have any questions, tag [Honza Dvorsky](https://github.com/czechboy0) or
 - <doc:SOAR-0001>
 - <doc:SOAR-0002>
 - <doc:SOAR-0003>
+- <doc:SOAR-0004>