feature: Add support for interface default methods

[MariusVolkhart]

Is your feature request related to a problem? Please describe.
I have a service that exposes an API that isn't very ergonomic. We see callers repeatedly writing the same 5 lines to interact with the service. We'd like to make it easier by adding a default method to our Refit interface that does the logic of those 5 lines, which includes calling a Refit-annotated method.

interface Mine {
  [Get("/events.json")]
  Task<Events> GetEvents(string query);
  
  Task<Events> GetEventsOrDefault(string query) {
    try {
      GetEvents(query);
    } catch (ApiException e) {
        // Look at exception for Reasons
    }
}

Describe the solution you'd like
Right now, Refit errors with the message, "System.NotImplementedException : Either this method has no Refit HTTP method attribute or you've used something other than a string literal for the 'path' argument."

#845 Had a fix for static default methods, but it does not appear to address default instance methods.

Describe alternatives you've considered
For now we're using an extension method

[MariusVolkhart]

🥳 thanks for doing this @clairernovotny!

[github-actions]

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