Support for Sending Text/Image Attachment Content

Added expand_text_attachments and enable_image_comprehension settings to request parsed content/descriptions from text and image attachments with the query request. This content is sent through the new parsed_content field in the attachment dictionary. This change makes enabling file uploads much simpler.
Added the enforce_author_role_alternation setting. If enabled, Poe will concatenate messages so that they follow role alternation, which is a requirement for certain LLM providers like Anthropic.