create TLS client & server with full set of arguments

[CodiePP]

Signed-off-by: Alexander Diemand codieplusplus@apax.net

This change allows to pass in more arguments to the basic Tls_io functions to setup its Tls.Config.

This allows to more finegrained control the TLS handshake and improve security.

An example of its usage is in: https://github.com/CodiePP/ml-grpc-examples
(an accompanying PR is submitted to https://github.com/anmonteiro/ocaml-h2)

[anmonteiro]

My rationale for having a limited set of options for these default handlers is that you can always "jailbreak" by using your own Tls_lwt.Unix.t, check out this example https://github.com/anmonteiro/ocaml-h2/blob/fceaad2cfa198f090b47d6d72cb1c4ea9dee4a07/examples/alpn/unix/alpn_server_tls.ml#L29-L48

[CodiePP]

My rationale for having a limited set of options for these default handlers is that you can always "jailbreak" by using your own Tls_lwt.Unix.t, check out this example https://github.com/anmonteiro/ocaml-h2/blob/fceaad2cfa198f090b47d6d72cb1c4ea9dee4a07/examples/alpn/unix/alpn_server_tls.ml#L29-L48

true. Since the arguments are optional they could be put on the default handler too.