add SshInteractive credential

In reference to https://libgit2.github.com/libgit2/#HEAD/group/cred/git_cred_ssh_interactive_new

It would be great to have this implemented so I can fetch the passphrase for an ssh key and allow libgit2 to ask for the passphrase.

Note: this does not include a default prompt_callback. Which I think would be great to add since many people will probably struggle with this.

I would make a PR but I have no idea how to call the associated libgit methods.

module Rugged
  module Credentials
    class SshInteractive
      def initialize(username, prompt_callback)
        @username = username
        @prompt_callback = prompt_callback
      end

      def call(url, username_from_url, allowed_types)
        self
      end
    end
  end
end

Start with the referenced libgit2 git_cred_ssh_interactive_new API and the Rugged::Credentials::SshInteractive entry point shown in the issue. Compare the surrounding credential implementations, then verify that fetching can obtain an SSH passphrase through the supplied prompt_callback and that the credential is accepted by libgit2.