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/image.rs

@@ -0,0 +1,65 @@
+use mxlink::mime;
+
+#[derive(Default)]
+pub struct ImageGenerationParams {
+    pub size_override: Option<String>,
+
+    pub cheaper_model_switching_allowed: bool,
+
+    pub cheaper_quality_switching_allowed: bool,
+}
+
+impl ImageGenerationParams {
+    pub fn with_size_override(mut self, value: Option<String>) -> Self {
+        self.size_override = value;
+        self
+    }
+
+    pub fn with_cheaper_model_switching_allowed(mut self, value: bool) -> Self {
+        self.cheaper_model_switching_allowed = value;
+        self
+    }
+
+    pub fn with_cheaper_quality_switching_allowed(mut self, value: bool) -> Self {
+        self.cheaper_quality_switching_allowed = value;
+        self
+    }
+}
+
+pub struct ImageGenerationResult {
+    pub bytes: Vec<u8>,
+    pub mime_type: mime::Mime,
+    pub revised_prompt: Option<String>,
+}
+
+#[derive(Default)]
+pub struct ImageEditParams {
+}
+
+pub struct ImageEditResult {
+    pub bytes: Vec<u8>,
+    pub mime_type: mime::Mime,
+}
+
+pub struct ImageSource {
+    pub filename: String,
+    pub bytes: Vec<u8>,
+    pub mime_type: mime::Mime,
+}
+
+impl ImageSource {
+    pub fn new(filename: String, bytes: Vec<u8>, mime_type: mime::Mime) -> Self {
+        Self { filename, bytes, mime_type }
+    }
+}
+
+impl Into<async_openai::types::ImageInput> for ImageSource {
+    fn into(self) -> async_openai::types::ImageInput {
+        async_openai::types::ImageInput{
+            source: async_openai::types::InputSource::VecU8 {
+                filename: self.filename,
+                vec: self.bytes,
+            },
+        }
+    }
+}