feature: tcpsock sslhandshake alpn

[bzp2010]

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

[theweakgod]

The str_byte loop works, but we can simplify this quite a bit.

Since ALPN identifiers are a finite IANA-registered set, a pre-built wire-format lookup avoids runtime byte construction entirely for common protocols, with a fallback for anything else:

local ALPN_WIRE = {
    ["h2"]       = "\x02h2",
    ["http/1.1"] = "\x08http/1.1",
    ["http/1.0"] = "\x08http/1.0",
}

local bytes_array = {}
for _, p in ipairs(alpn) do
    bytes_array[#bytes_array+1] = ALPN_WIRE[p] or (string.char(#p) .. p)
end
local _bytes = table.concat(bytes_array)
alpn_str[0].data = ffi.new("unsigned char[?]", #_bytes, _bytes)
alpn_str[0].len  = #_bytes

table.concat produces the full wire string in one allocation