Use patched async-openai library to work around a few upstream issues

Related to:

- CreateImageEditRequest forces an application/octet-stream content type
  for images (https://github.com/64bit/async-openai/issues/364)

- CreateImageEditRequest only deals with a single image
  (https://github.com/64bit/async-openai/issues/363)

This is a continuation of 8f86289373
and fixes the Image Editing feature for OpenAI.

Cargo.toml

@@ -17,7 +17,7 @@ path = "src/lib.rs"
 [dependencies]
 anthropic = "=0.0.8"
 anyhow = "1.0.*"
-async-openai = "0.28.*"
+async-openai = { git = "https://github.com/etkecc/async-openai", branch = "async-openai-v0.28.1-patched" }
 base64 = "0.22.*"
 chrono = { version = "0.4.*", default-features = false, features = ["std", "now"] }
 # We'd rather not depend on this, but we cannot use the ruma-events EventContent macro without it.