Add Client with response wrapper, request helpers, and full middleware assembly

Implements the top-level httpx.Client that composes the full chain:
  Logging → User Middlewares → Retry → Circuit Breaker → Balancer → Transport

- Response wrapper with JSON/XML/Bytes decoding and body caching
- NewJSONRequest helper with Content-Type and GetBody support
- Functional options: WithBaseURL, WithTimeout, WithRetry, WithEndpoints, etc.
- Integration tests covering retry, balancing, error mapping, and JSON round-trips

request.go

@@ -0,0 +1,34 @@
+package httpx
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+// NewRequest creates an http.Request with context. It is a convenience
+// wrapper around http.NewRequestWithContext.
+func NewRequest(ctx context.Context, method, url string, body io.Reader) (*http.Request, error) {
+	return http.NewRequestWithContext(ctx, method, url, body)
+}
+
+// NewJSONRequest creates an http.Request with a JSON-encoded body and
+// sets Content-Type to application/json.
+func NewJSONRequest(ctx context.Context, method, url string, body any) (*http.Request, error) {
+	b, err := json.Marshal(body)
+	if err != nil {
+		return nil, fmt.Errorf("httpx: encoding JSON body: %w", err)
+	}
+	req, err := http.NewRequestWithContext(ctx, method, url, bytes.NewReader(b))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.GetBody = func() (io.ReadCloser, error) {
+		return io.NopCloser(bytes.NewReader(b)), nil
+	}
+	return req, nil
+}