Add q-factor parsing feature

[timothy-jeong]

This PR introduces optional support for q-factor (quality value) negotiation for the Accept-Encoding header, aligning the middleware's behavior more closely with RFC 9110 (Section 12.5.3).

Key Changes

• New Flag: Introduces a respect_q_factors: bool = False argument to the ZstdMiddleware constructor.
• Backward Compatibility: The default behavior (False) is unchanged, ensuring zero performance impact or breaking changes for existing users.
• Opt-In Logic: When respect_q_factors=True, the middleware uses a new header-parsing utility to select the best encoding (zstd, gzip, or identity) based on the client's q values.
• Performance: The new get_preferred_encoding function is cached with @lru_cache(maxsize=128) to mitigate any performance overhead from parsing the same header strings on subsequent requests.

Resolve: #12