MEDIUM: httpclient/lua: allow multiple requests from a single core.httpclient() instance

MEDIUM: httpclient/lua: allow multiple requests from a single core.httpclient() instance

Refactor the Lua HTTP client to defer initialization. core.httpclient()
no longer initializes the internal HTTP client immediately. Instead,
initialization now occurs within hlua_httpclient_send() when a request
method (e.g., get, put, head) is invoked.

The HTTPClient class now serves as a factory for accessing methods, while
a new class, HTTPClientRequest, has been introduced to represent individual
requests and manage the HTTP client lifecycle.

This change allows multiple requests to be executed using a single
HTTP client instance:

  local hc = core.httpclient()
  local res1 = hc:get({url = "...", headers = ...})
  local res2 = hc:post({url = "...", headers = ...})
  local res3 = hc:put({url = "...", headers = ...})

This refactor maintains backward compatibility, as existing scripts that
instantiate a new core.httpclient() for every request will continue to
work as expected.