[Feature] http: optional insertion-ordered header emission

[Feature] http: optional insertion-ordered header emission

The HTTP client stores headers in a khash and emits them in bucket
order, so the on-the-wire header order is unpredictable. Add an opt-in
RSPAMD_HTTP_FLAG_ORDERED_HEADERS flag: each header is stamped with a
monotonic `order` at insertion time, and when the flag is set the
client serialises headers sorted by that order instead of hash order.

lua_http now accepts a list form for the headers table
({{'name', 'value'}, ...}) which preserves order and sets the flag;
the existing map form and every other caller are byte-identical.

This lets callers reproduce a real browser's exact header order, used
by the url_redirector stealth fingerprint profiles.