Add middlebox_comp_mode to global options

[elbrujohalcon]

This PR adds another option (like ssl_verify) to the global rebar.config: middlebox_comp_mode.
This is needed to address issues like #2987.
The option is basically passed down as-is (with the default being true) to httpc:request/5 where it's used as described in the docs. That is…

{middlebox_comp_mode, MiddleBoxMode}

Configures the middlebox compatibility mode for a TLS-1.3 connection.

A significant number of middleboxes misbehave when a TLS-1.3 connection is negotiated. Implementations can increase the chance of making connections through those middleboxes by adapting the TLS-1.3 handshake to resemble that of a TLS-1.2 handshake.

The middlebox compatibility mode is enabled (true) by default.

[ferd]

Makes sense as an extra option; re-running failed MacOS suite to see if it was transient.