Add SOAR-0007 proposal: Shorthand APIs for inputs and outputs (#291)

### Motivation

We'd like to run a proposal for generating additional API to simplify
providing operation inputs and handling
operation outputs for simple API calls with just one successful outcome.

### Modifications

- Add SOAR-0007: Shorthand APIs for operation inputs and outputs

(See the proposal itself for details)[^1]

### Result

n/a

### Test Plan

n/a

### Related Issues

#22, #104, #105, #145

[^1]: https://github.com/apple/swift-openapi-generator/pull/291/files

---------

Signed-off-by: Si Beaumont <beaumont@apple.com>

Author: simonjbeaumont

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

@@ -48,3 +48,4 @@ If you have any questions, tag [Honza Dvorsky](https://github.com/czechboy0) or
 - <doc:SOAR-0003>
 - <doc:SOAR-0004>
 - <doc:SOAR-0005>
+- <doc:SOAR-0007>