Add a soundness --fix flag

### Motivation

When running `./scripts/soundness.sh` produces swift-format warnings, we ask adopters to manually copy/paste a call to swift format to fix the warnings up. This is tedious and unnecessary.

### Modifications

Add a `--fix` option on the `soundness.sh` script to actually apply the fixes as well, avoiding the need to copy/paste long commands.

### Result

Easier fixing up of formatting warnings.

### Test Plan

Manually tested the workflow locally.


Reviewed by: glbrntt

Builds:
     ✔︎ pull request validation (5.10) - Build finished. 
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (api breakage) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#62

[Runtime] Generate server variables

### Motivation

The runtime side of apple/swift-openapi-generator#24.

### Modifications

Added an SPI `ServerVariable` type and a variant of the `URL.init(validatingOpenAPIServerURL:variables:)` method, which takes the template and variables and returns a fully formed concrete URL.

### Result

Unblocked the generator half of supporting server variables.

### Test Plan

Added unit tests.


Reviewed by: glbrntt

Builds:
     ✔︎ pull request validation (5.10) - Build finished. 
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (api breakage) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#64

#63)

Improved enriched error propagation from the transport and middlewares

### Motivation

Fixes apple/swift-openapi-generator#302 and apple/swift-openapi-generator#17.

The issue was that we hid away errors thrown in transports and middlewares, and the adopter would get `ClientError` where the `underlyingError` wasn't the error thrown by the underlying transport/middleware, but instead a private wrapper of ours.

### Modifications

Make sure `{Client,Server}Error.underlyingError` contains the error thrown from the underlying transport/middleware when that was the cause of the error, otherwise keep `RuntimeError` there as was the behavior until now.

Also added a `causeDescription` property on both public error types to allow communicating the context for the underlying error.

Also made sure middleware errors are now wrapped in Client/ServerError, they weren't before so didn't contain the context necessary to debug issues well.

### Result

Adopters can now extract the errors thrown e.g. by URLSession from our public error types using the `underlyingError` property and understand the context of where it was thrown by checking the user-facing `causeDescription`.

Also, adopters now get enriched errors thrown from middlewares.

### Test Plan

Wrote unit tests for both UniversalClient and UniversalServer, inevitably found some minor bugs there as well, fixed them all, plus the unit tests now verify the behavior new in this PR.


Reviewed by: glbrntt

Builds:
     ✔︎ pull request validation (5.10) - Build finished. 
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 
     ✖︎ pull request validation (api breakage) - Build finished. 

#63