feat(server): add functional options for BlossomServer configuration

Introduce `ServerOption` to configure `BlossomServer` with functional
options. Add `WithRedirectURL` to enable flexible redirect URL handling
with placeholders. Update `New` constructor to accept optional
configurations.

blossom/server.go

@@ -26,11 +26,29 @@ type BlossomServer struct {
 	RejectDelete []func(ctx context.Context, auth *nostr.Event, sha256 string) (bool, string, int)
 }
 
-func New(rl *khatru.Relay, serviceURL string) *BlossomServer {
+// ServerOption represents a functional option for configuring a BlossomServer
+type ServerOption func(*BlossomServer)
+
+// WithRedirectURL configures a redirect URL for the RedirectGet function
+func WithRedirectURL(urlTemplate string, statusCode int) ServerOption {
+	return func(bs *BlossomServer) {
+		redirectFn := redirectGet(urlTemplate, statusCode)
+		bs.RedirectGet = append(bs.RedirectGet, redirectFn)
+	}
+}
+
+// New creates a new BlossomServer with the given relay and service URL
+// Optional configuration can be provided via functional options
+func New(rl *khatru.Relay, serviceURL string, opts ...ServerOption) *BlossomServer {
 	bs := &BlossomServer{
 		ServiceURL: serviceURL,
 	}
 
+	// Apply any provided options
+	for _, opt := range opts {
+		opt(bs)
+	}
+
 	base := rl.Router()
 	mux := http.NewServeMux()
 
@@ -85,3 +103,40 @@ func New(rl *khatru.Relay, serviceURL string) *BlossomServer {
 
 	return bs
 }
+
+// redirectGet returns a function that redirects to a specified URL template with the given status code.
+// The URL template can include {sha256} and/or {extension} placeholders that will be replaced
+// with the actual values. If neither placeholder is present, {sha256}.{extension} will be
+// appended to the URL with proper forward slash handling.
+func redirectGet(urlTemplate string, statusCode int) func(context.Context, string, string) (url string, code int, err error) {
+	return func(ctx context.Context, sha256 string, extension string) (string, int, error) {
+		finalURL := urlTemplate
+
+		// Replace placeholders if they exist
+		hasSHA256Placeholder := strings.Contains(finalURL, "{sha256}")
+		hasExtensionPlaceholder := strings.Contains(finalURL, "{extension}")
+
+		if hasSHA256Placeholder {
+			finalURL = strings.Replace(finalURL, "{sha256}", sha256, -1)
+		}
+
+		if hasExtensionPlaceholder {
+			finalURL = strings.Replace(finalURL, "{extension}", extension, -1)
+		}
+
+		// If neither placeholder is present, append sha256.extension
+		if !hasSHA256Placeholder && !hasExtensionPlaceholder {
+			// Ensure URL ends with a forward slash
+			if !strings.HasSuffix(finalURL, "/") {
+				finalURL += "/"
+			}
+
+			finalURL += sha256
+			if extension != "" {
+				finalURL += "." + extension
+			}
+		}
+
+		return finalURL, statusCode, nil
+	}
+}