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/agent/provider/entity/mod.rs

@@ -1,12 +1,12 @@
 mod agent_provider;
-mod image_generation;
+mod image;
 mod ping;
 mod speech_to_text;
 mod text_generation;
 mod text_to_speech;
 
 pub use agent_provider::{AgentProvider, AgentProviderInfo};
-pub use image_generation::{ImageGenerationParams, ImageGenerationResult};
+pub use image::{ImageGenerationParams, ImageGenerationResult, ImageEditParams, ImageEditResult, ImageSource};
 pub use ping::PingResult;
 pub use speech_to_text::{SpeechToTextParams, SpeechToTextResult};
 pub use text_generation::{