Implements retry middleware as a RoundTripper wrapper:
- Exponential and constant backoff strategies with jitter
- RFC 7231 Retry-After header parsing (seconds and HTTP-date)
- Default policy retries idempotent methods on 429/5xx and network errors
- Body restoration via GetBody, context cancellation, response body cleanup
Introduce the core building blocks for the httpx library:
- middleware.Middleware type and Chain() composer
- Error struct with sentinel errors (ErrRetryExhausted, ErrCircuitOpen, ErrNoHealthy)
- internal/clock package with Clock interface and MockClock for deterministic testing
