Initial work on Vision support in text conversations and Image Editing

This is a huge patch which does some major refactoring like:

- renaming "Image Generation" to "Image Creation" in most places,
  to better match its new command (`!bai image create`)

- relocating image creation command (`!bai image` -> `!bai image create`),
  so it wouldn't conflict with the new image editing command (`!bai image edit`)

- introducing a new image editing command (`!bai image edit`), which
  is meant to work only with the OpenAI provider, but doesn't fully work yet
  due to https://github.com/64bit/async-openai/issues/364, though a next patch will fix it

- adding support for reading images off of Matrix conversations and forwarding them to
  text conversations. Works for OpenAI, but not for Anthropic yet
  (requires custom patches) and not for OpenAI-Compat (no support for
  images there)

- relocating some utils around (base64, mime)

src/conversation/matrix/entity.rs

@@ -2,20 +2,22 @@ use chrono::{DateTime, Utc};
 use regex::Regex;
 
 use mxlink::matrix_sdk::ruma::OwnedUserId;
+use mxlink::matrix_sdk::ruma::events::room::message::ImageMessageEventContent;
+use mxlink::mime::Mime;
 
 #[derive(Clone)]
 pub struct MatrixMessage {
     pub sender_id: OwnedUserId,
-    pub message_type: MatrixMessageType,
-    pub message_text: String,
+    pub content: MatrixMessageContent,
     pub mentioned_users: Vec<OwnedUserId>,
     pub timestamp: DateTime<Utc>,
 }
 
 #[derive(Clone)]
-pub enum MatrixMessageType {
-    Text,
-    Notice,
+pub enum MatrixMessageContent {
+    Text(String),
+    Notice(String),
+    Image(ImageMessageEventContent, Mime, Vec<u8>),
 }
 
 #[derive(Clone)]