Add :supported_groups to @mint_tls_opts

[numso]

Hello!

tldr; would you be open to a PR that adds :supported_groups to the list of @mint_tls_opts?

---

I ran into an issue yesterday while trying to configure SSL in Finch. I'm working with a client whose server is misconfigured and is rejecting any SSL handshake that contains :brainpoolP256r1tls13 in the supported_groups for key exchange. Erlang's :ssl module has an option for adjusting the supported_groups.

I'm able to add that option to Finch like so:

config :my_app, Finch,
  name: MyApp.Finch,
  pools: %{
    default: [
      conn_opts: [
        transport_opts: [
          supported_groups: :ssl.groups(:default) -- [:brainpoolP256r1tls13]
        ]
      ]
    ]
  }

This fixes my first problem but introduces a second: Requests to http (not https) origins now fail with a badarg error.

It looks Finch maintains a list of SSL options that should be removed when making an http request. Because :supported_groups is missing from that list, it is passed as an option for http requests which causes the badarg error:

finch/lib/finch/pool_manager.ex

Lines 5 to 20 in c3b9411

	@mint_tls_opts [
	:cacertfile,
	:ciphers,
	:depth,
	:eccs,
	:hibernate_after,
	:partial_chain,
	:reuse_sessions,
	:secure_renegotiate,
	:server_name_indication,
	:signature_algs,
	:signature_algs_cert,
	:verify,
	:verify_fun,
	:versions
	]