Overriding Net::SSH::Config.default_files is not the best strategy as of today

Hi, I'm currently unable to use the ProxyJump config with sshkit, due to this issue:

https://github.com/net-ssh/net-ssh/issues/587

sshkit will override Net::SSH.default_files here:

https://github.com/capistrano/sshkit/blob/158c2867071e015a117e188d1a0ee7b579c9b24e/lib/sshkit/backends/netssh.rb#L13

By the way, is there any reasons to do that instead of something simpler like:

```ruby
Net::SSH::Config.default_files << File.join(Dir.pwd, '.ssh/config')
```

Either way, net-ssh doesn't work the same way when the :config option is used in Net::SSH.start and when passed to default_files, as you could see in the issue I mentioned above.

Currently I'm in a rush to try to meet some tight deadlines for a new project I started and it will last a few months at least. If I were to find some time to contribute it would be more likely to provide some patch to sshkit to use the :config option instead of overriding `default_files` as it seems a simpler/quicker change for someone new to both projects, in my opinion. Before I do so, I'd like to check on your opinion. Is there anyone here that is comfortable with net-ssh source and would be willing to contribute to it by providing the "-F" option to the ssh client for each of those default_files? Otherwise, would you be willing to accept a patch that would change the current approach in sshkit so that this default configuration would be passed to the :config option instead of overriding `default_files`?

Thanks for your time to read all of this :)