Add flag to disable HTTP 2

[jb3]

Adds a new config flag force_http1 that explicitly sets the protocols in the
Gun open options to use HTTP1. By default, if a TLS connection is set in the
transport (which defaults to TLS if port 443 is used), then the protocols
defaults to [:http2, :http], preferring HTTP2.

We are currently trying to diagnose an issue with stagnation of connections in
the ratelimiter due to missed responses and this flag would allow us to rule out
HTTP2 as having influence in the problems.

I have tested this locally, as you can see here the first map is an inspected
debug log of the generated options and the response is received as normal:

iex(1)> Nostrum.Api.get_user 165023948638126080
%{
  ...
  protocol: :http,
  transport: :tls,
  ...
  origin_host: ~c"discord.com",
  origin_port: 443,
  origin_scheme: "https",
  ...
}
{:ok,
 %Nostrum.Struct.User{
   id: 165023948638126080,
   username: "joe.banks",
   discriminator: "0",
   global_name: "joe",
   avatar: "a_bbe3b1029f3ab24d8034bebd9772ed2c",
   bot: nil,
   public_flags: 4457216
 }}

[jchristgit]

Thanks!