refactor: rewrite entire library to use HttpClient trait instead of reqwest::Client

- Replace hardcoded reqwest::Client with HttpClient trait throughout
- Client struct now accepts any HttpClient implementation
- Remove duplicate ClientWithTrait and ChatWithTrait structures
- All API methods (get, post, delete) now use the trait
- Backward compatibility: forms and streaming still use reqwest directly (TODO)
- Default implementation uses reqwest::Client for zero breaking changes
- This enables middleware injection for tracing, logging, etc.

Author: timvw

async-openai/Cargo.toml

@@ -28,6 +28,7 @@ byot = []
 
 [dependencies]
 async-openai-macros = { path = "../async-openai-macros", version = "0.1.0" }
+async-trait = "0.1"
 backoff = { version = "0.4.0", features = ["tokio"] }
 base64 = "0.22.1"
 futures = "0.3.31"
@@ -40,6 +41,7 @@ reqwest = { version = "0.12.12", features = [
 reqwest-eventsource = "0.6.0"
 serde = { version = "1.0.217", features = ["derive", "rc"] }
 serde_json = "1.0.135"
+serde_urlencoded = "0.7"
 thiserror = "2.0.11"
 tokio = { version = "1.43.0", features = ["fs", "macros"] }
 tokio-stream = "0.1.17"