feature: added the ngx.configure module and APIs

[thibaultcha]

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-resty-core project.

Sister PR of openresty/lua-nginx-module#1259

FFI-based API for dynamic NGINX configuration via a new configure_by_lua phase.

Synopsis:

http {
    configure_by_lua_block {
        local ngx_configure = require "ngx.configure"

        -- hard-coded, but could be retrieved from I/O or env
        local shms = {
            { "dogs", "12k" },
            { "cats", "24k" }
        }

        -- create shms
        for _, shm in ipairs(shms) do
            ngx_configure.shared_dict(shm[1], shm[2])
        end

        -- conditionally expose env variables for workers
        ngx_configure.env("PROXY_PASS_URL")

        -- configure the Lua VM
        ngx_configure.max_pending_timers(1024)
        ngx_configure.max_running_timers(128)
    }

    init_by_lua_block {
        ngx.shared.dogs:set("hello", true)
    }
}