sanitize transport options

[aymanosman]

Problem

Some transport options are invalid when passed to the tcp transport,
but machine_gun pools are specified not for a specific host-port pair
but for a so called "pool group", this means that options set for the
ssl transport are being passed to the tcp transport and causing
failure.

Solution

Drop invalid options depending on the given transport.
Currently only verify, as that is the option that is being specified.

[aymanosman]

Demo of the problem

First install gun

> Mix.install [:gun]
:ok

Passing invalid arguments to the tcp transport raises an error

> {:ok, port} = :gen_tcp.connect('httpbin.org', 80, [verify: :verify_none])
** (exit) :badarg
    (kernel 8.2) gen_tcp.erl:172: :gen_tcp.connect/4

gun swallows the error, the process has terminated but you only notice this once you make a request.

> {:ok, pid} = :gun.open('httpbin.org', 80, %{transport: :tcp, transport_opts: [verify: :verify_none]})
{:ok, #PID<0.220.0>}
> Process.alive? pid
false