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/entity/message_payload.rs

@@ -1,5 +1,5 @@
 use mxlink::matrix_sdk::ruma::events::room::message::{
-    AudioMessageEventContent, MessageType, TextMessageEventContent,
+    AudioMessageEventContent, ImageMessageEventContent, MessageType, TextMessageEventContent,
 };
 use mxlink::matrix_sdk::ruma::{OwnedEventId, OwnedUserId};
 
@@ -29,6 +29,7 @@ pub enum MessagePayload {
 
     Text(TextMessageEventContent),
     Audio(AudioMessageEventContent),
+    Image(ImageMessageEventContent),
 
     Reaction {
         key: String,
@@ -55,6 +56,7 @@ impl TryInto<MessagePayload> for MessageType {
                 // For this reason, we handle all audio.
                 MessagePayload::Audio(audio_content)
             }
+            MessageType::Image(image_content) => MessagePayload::Image(image_content),
             other => {
                 return Err(format!("Unsupported message type: {:?}", other));
             }