feat(cache): add build_cache_key() for custom key builders

[allen0099]

Summary

Adds build_cache_key(request, *components) (exported from fastapi_cachex) so a custom key_builder can add a dimension (user ID, tenant, locale) without rebuilding method|||host|||path|||query by hand.

from fastapi_cachex import build_cache_key

def per_user_key(request: Request) -> str:
    return build_cache_key(request, request.state.user_id)

• Additive: with no components it returns exactly the previous default key (pinned by a test against the old formula for plain, query, escaped host/path, missing host and HEAD requests). default_key_builder stays public and returns build_cache_key(request).
• Component typing: str | int. An int is written in decimal (1 and "1" are the same component). Anything else, including None and bool, raises TypeError, so a missing user ID cannot silently put every such caller under a "None" key. An empty string is a component of its own.
• Escaping: every component goes through escape_key_component, so "a|||b" cannot line up with the components "a", "b".
• clear_path: extra components follow the query string. Memory's key splitter used to split at most three times, so GET|||h|||/me||||||user counted as "has query params" and clear_path("/me") missed it; it now looks at the query component only. Redis now SCANs *|||path|||* and checks each returned key's third component and query in Python (a glob cannot tell an empty query followed by extras from a query). Side effect: a key where the path only appears as the host or an extra component is no longer cleared. Memcached is unchanged (exact-key only). A contract test runs on memory and live Redis.
• Monitoring routes: /cached-hits and /cached-records records get a new extra_components list (decoded); query_params now holds only the query string. _parse_cache_key keeps its 4-tuple shape.
• Docs: new "Adding components to the key" section in HTTP_CACHING.md, the "Authenticated endpoints" per-user example now uses build_cache_key, CACHE_FLOW.md points to it, API reference lists it; zh-TW mirrors updated.

Changelog

changelog.d/264.added.md

Test plan

• uv run ruff check fastapi_cachex tests && uv run ruff format --check fastapi_cachex tests && uv run mypy fastapi_cachex --strict
• Full suite including the live Redis and Memcached tests: 1148 passed, 1 skipped, coverage 99%
• uv run pytest (no live servers): 957 passed, 192 skipped, coverage 94%

Closes #264