Support Hackney 2.x

[tomekowal]

This PR is a starting point on supporting hackney 2.x by tesla.
So far:

• updated the adapter to support connection handle being either Ref (in 1.x) or Pid (in 2.x)
• Discovered a test failing because of a change in Hackney API that is not covered by the migration guide and opened issue about it hackney:send_body does not accept functions anymore benoitc/hackney#821

TBD:

• if the adapter for Hackney 2.x will require extensive changes, it might be better to have a second module and decinde at runtime which one to load

TODO:

• make sure all tests pass
• squash to conventional commits after everything works

Note: the tests here are green because they still use old hackney. On new version

[yordis]

if the adapter for Hackney 2.x will require extensive changes, it might be better to have a second module and decinde at runtime which one to load

If you feel that is the right approach, I really do not mind. These type of configuration are barely the "every day" situation. So, feel free to create a HackneyV2 and we mark the module as experimental in the worse case.

Try to keep safety checks at compile-time as much as we can.

Make it work, then make it beautiful.