Enable strict concurrency checking for NIOHTTP1Client (#3131)

### Motivation:

To ensure `NIOHTTP1Client` concurrency safety.

### Modifications:

* Enable strict concurrency checking in the package manifest.

### Result:

Builds will warn and CI will fail if regressions are introduced.

Author: rnro

Package.swift

@@ -330,7 +330,8 @@ let package = Package(
                 "NIOHTTP1",
                 "NIOConcurrencyHelpers",
             ],
-            exclude: ["README.md"]
+            exclude: ["README.md"],
+            swiftSettings: strictConcurrencySettings
         ),
         .executableTarget(
             name: "NIOChatServer",