@cache: public build_cache_key() helper so custom key builders don't hand-roll the format

[allen0099]

Problem

A custom key_builder that adds a dimension (user ID, tenant, locale) has to rebuild the whole default key by hand. The per-user example in HTTP_CACHING.md did that and repeated #230: it joined the raw Host header and path with |||. Every hand-rolled builder is one forgotten escape_key_component away from the same key-poisoning bug.

Proposal

Add a public helper that builds the default key and appends extra components, encoding every component:

from fastapi_cachex import build_cache_key

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

• default_key_builder(request) becomes build_cache_key(request).
• Extra components go through escape_key_component, so they cannot inject the separator either.
• Update the docs example to use it.

Additive; no key changes for existing routes.