Support to call API file enpoint

[JuanKman94]

Has there been a discussion about adding support for the file method?

Telegram::Bot::Api already supports getFile so adding a method to download the file afterwards makes sense. Looking at the code I was thinking of two low-hanging solutions that can be implemented:

1. Exposing the Faraday connection and let the caller use it
2. Adding a #file method

IMO option 1 makes for messier code as the caller would need to do something like this to build the URL.

photo = select_photo(update)
get_file_response = client.api.get_file(file_id: photo.file_id)
file_response = client.conn.get("/file/bot#{client.api.token}/#{get_file_response.dig("result", "file_path")}")

store_file(file_response.body)

While adding a specific method would let Telegram::Bot::Api do the setup and the caller would have a nice interface. Maybe something like this:

class Telegram::Bot::Api
  def file(file_path)
    response = conn.get("/file/bot#{token}/#{file_path}")
    unless response.status == 200
      raise Exceptions::ResponseError.new(response), 'Telegram API has returned an error.'
    end

    response.body
  end
end

If maintainers are busy I can get a PR up.

[JuanKman94]

Proposed implementation in #269