File not found middleware for the Website service (#4)

This PR contains the work done to add a `NotFoundMiddleware` middleware to the Website service so requests matching neither a route nor a static file return a custom 404.html page with a 404 Not Found status. Also enables index.html fallback so the landing page is served at the site root.

To provider further details about the work done:
* NotFoundMiddleware — intercepts the `.notFound` error from the  `FileMiddleware` middleware and serves the preloaded error page with the correct content type; all other errors propagate. Falls back to a minimal body if the file is missing.
* Router — wires the `NotFoundMiddleware` middleware ahead of the `FileMiddleware` and enables searchForIndexHtml.
* Package — adds the **Hummingbird** product to the Library target.
* Tooling — sets a custom working directory in the Xcode scheme; removes the unused `pkg-deps` target from the `Makefile` file.

Reviewed-on: rock-n-code/loud-amsterdam#4
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/Library/Public/Middlewares/NotFoundMiddleware.swift

@@ -0,0 +1,80 @@
+import Foundation
+import Hummingbird
+import NIOCore
+
+/// Serves a custom error page for requests that match neither a route nor a static file.
+///
+/// Placed ahead of `FileMiddleware` in the middleware chain, it catches the `.notFound` error
+/// that bubbles up when no file exists for the requested path and responds with the preloaded
+/// error page and a `404 Not Found` status.
+public struct NotFoundMiddleware<Context: RequestContext> {
+    
+    // MARK: Properties
+    
+    /// The body of the error page served on a not-found response.
+    private let page: ByteBuffer
+    
+    // MARK: Initializers
+    
+    /// Creates a middleware that serves the error page (`404.html`) from the static files folder.
+    ///
+    /// The page is read once, at construction. A minimal fallback body is used when the file is
+    /// missing.
+    /// - Parameter staticFilesPath: the folder, relative to the working directory, the static files are served from.
+    public init(
+        _ staticFilesPath: String
+    ) {
+        let path = StaticFile.errorHTML.path(relativeTo: staticFilesPath)
+        
+        if let data = try? Data(contentsOf: URL(fileURLWithPath: path)) {
+            self.init(page: .init(bytes: data))
+        } else {
+            self.init(page: .init(string: "404 Not Found"))
+        }
+    }
+    
+    /// Creates a middleware that serves the given error page on a not-found response.
+    /// - Parameter page: the body of the error page.
+    init(
+        page: ByteBuffer
+    ) {
+        self.page = page
+    }
+    
+}
+
+// MARK: - RouterMiddleware
+
+extension NotFoundMiddleware: RouterMiddleware {
+
+    // MARK: Functions
+
+    public func handle(
+        _ request: Request,
+        context: Context,
+        next: (Request, Context) async throws -> Response
+    ) async throws -> Response {
+        do {
+            return try await next(request, context)
+        } catch let error {
+            // Only intercept "not found"; let every other error propagate.
+            guard
+                let responseError = error as? any HTTPResponseError,
+                responseError.status == .notFound
+            else {
+                throw error
+            }
+
+            var headers = HTTPFields()
+
+            headers[.contentType] = StaticFile.errorHTML.contentType
+
+            return Response(
+                status: .notFound,
+                headers: headers,
+                body: .init(byteBuffer: page)
+            )
+        }
+    }
+
+}