feat(cache)!: normalise the host in HTTP cache keys

[allen0099]

Third of the 0.4.0 cache-key changes, after #405 and #406.

What changes

The host component of an HTTP cache key is normalised, so every spelling of one origin shares an entry:

• lower-cased (RFC 9110 §4.2.3);
• an empty port or the scheme's default one dropped (:80 on http/ws, :443 on https/wss; RFC 3986 §6.2.3). The scheme is read from the ASGI scope, which is also what request.url uses; request.url.scheme itself comes back empty for some malformed hosts;
• IPv6 literals keep their brackets ([fe80::1]:8000);
• a trailing dot is not stripped, a missing Host still gives unknown, and a malformed host (unbracketed IPv6, :80, host:80:80) is kept as sent.

@cache, build_cache_key(), invalidate() and the monitoring routes all go through CacheKey.from_request, so they agree. clear_path() wildcards the host.

Docs

• HTTP_CACHING.md and CACHE_FLOW.md (English and zh-TW) describe the rules, including that behind a TLS-terminating proxy the scheme is http unless the proxy's headers are applied.
• MIGRATING_0_4.md states the rule precisely and notes that clear_pattern() patterns naming an upper-case host or a default port need rewriting.
• changelog.d/265.changed.md.

Tests

tests/test_host_normalization.py:

• 23 host/scheme cases;
• an end-to-end check that EXAMPLE.com, example.com:80 and example.com hit one entry;
• invalidate() finding the entry under another spelling.

Closes #265