[AHCTransport] Consistent style for initializing local variables
### Motivation
Move to a consistent style when initializing local variables, always use `let foo = Foo(...)` vs `let foo: Foo = .init(...)`.
### Modifications
Updated all occurrences of the latter to use the former.
### Result
Consistent local variable initialization.
### Test Plan
All tests passed.
Reviewed by: gjcairo, simonjbeaumont
Builds:
✔︎ pull request validation (5.8) - Build finished.
✔︎ pull request validation (5.9) - Build finished.
✔︎ pull request validation (nightly) - Build finished.
✔︎ pull request validation (soundness) - Build finished.
https://github.com/swift-server/swift-openapi-async-http-client/pull/10