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/utils.rs

@@ -1,5 +1,3 @@
-use base64::{Engine as _, engine::general_purpose::STANDARD};
-
 use crate::{
     agent::{
         AgentInstance, AgentPurpose, ControllerTrait, Manager as AgentManager, PublicIdentifier,
@@ -140,7 +138,3 @@ async fn get_global_agent_id_for_purpose(
         .handler
         .get_by_purpose_with_catch_all_fallback(purpose)
 }
-
-pub(crate) fn base64_decode(base64_string: &str) -> Result<Vec<u8>, base64::DecodeError> {
-    STANDARD.decode(base64_string)
-}