feature: support cosocket and other yieldable APIs in init_worker_by_lua*

[tzssangglass]

Enable TCP/UDP cosockets, ngx.sleep, ngx.semaphore, ngx.thread.spawn, and coroutine.* inside init_worker_by_lua* by running a bounded event pump when the init code yields. Ported from ngx_http_lua_module and adapted to the stream subsystem.

Key changes:

• Add ngx_stream_lua_init_worker_pump() to drive the event loop during yields, including the posted delayed-events queue drain
• Add ngx_stream_lua_init_worker_pump_loop() with timeout budget management
• Add ngx_stream_lua_init_worker_toggle_accept() to disarm/re-arm listeners around the pump (prevents level-triggered busy-spin from pending accepts; mirrors nginx's reuseport/EPOLLEXCLUSIVE arming)
• Freeze ngx.timer.at/every registrations via ctx->context gate; flush at runner exit with absolute expiry preservation (timer ordering identical to before)
• Run the init chunk on an entry coroutine created with ngx_stream_lua_new_thread() on the global VM; finalize via finalize_fake_request on completion
• Add lua_init_worker_timeout directive (default 0 = no timeout)
• Add lua_init_worker_abort_on_error directive (default off)
• Add INIT_WORKER to NGX_STREAM_LUA_CONTEXT_YIELDABLE mask in util.h
• Change runtime error prefix to "lua entry thread aborted:" (breaking)
• Update README: new directives and cosocket availability note

sister PR: openresty/lua-nginx-module#2521