refactor(cache)!: add CacheKey as the one encoder and parser of HTTP keys

[allen0099]

Closes #270. This is step 1 of the 0.4.0 cache-key work (#271, #266, #265, #269, #72 follow).

What changes

• New public CacheKey (fastapi_cachex.CacheKey, module fastapi_cachex/cache_key.py), a frozen dataclass with:
  • from_request(request, *components, sort_query=...): the key @cache stores a request under;
  • to_str(): the single encoder;
  • parse(key): the single decoder. It returns None for keys that are not HTTP keys;
  • path_glob(path): the Redis glob for "every key with this path".
• build_cache_key() is now CacheKey.from_request(...).to_str(). The monitoring routes, MemoryBackend.clear_path and AsyncRedisCacheBackend.clear_path use CacheKey.parse / path_glob instead of splitting keys themselves.
• Removed: CACHE_KEY_MIN_PARTS, CACHE_KEY_MAX_SPLIT and CACHE_KEY_MAX_PARTS from fastapi_cachex.routes (maintainer decision: remove outright in 0.4.0).

The stored format does not change

Keys stay method|||host|||path|||query|||extra... and sort_query still defaults to False. A golden-string test pins the format, so the follow-up PRs change it on purpose. There are three small deltas:

• The method is percent-encoded like host and path. A method token may contain |. Without the encoding, build_cache_key() / invalidate() on such a request would produce an unparseable key, and with the new validation would raise. Real methods (GET, ...) are unaffected.
• CacheKey.query may not contain the separator (ValueError). A query taken from a request never does, because Starlette encodes |.
• clear_path compares the decoded path rather than the escaped one, and no longer treats a key with an empty method as an HTTP key. This makes no difference for keys written by build_cache_key. Only hand-written, non-canonically escaped keys could behave differently.

Tests and docs

• tests/test_cache_key_type.py: round trips, non-HTTP keys, escaping, validation, path_glob, the pinned format. Each assertion was checked by breaking the code it guards.
• Existing tests that imported routes._parse_cache_key / _split_cache_key now use CacheKey.parse.
• Docs: HTTP_CACHING.md (new CacheKey paragraph), MIGRATING_0_4.md, API reference, and the zh-TW counterparts.
• Changelog: changelog.d/270.added.md, changelog.d/270.removed.md.

Checks: pre-commit, mypy tests, mypy scripts, both strict docs builds, and the full suite with live Redis and Memcached (99.82% coverage).