feat: include reqwest in transport-streamble-http-client feature (#376)

* fix: add reqwest dependency to transport-streamable-http-client feature

- Fix compilation error when using transport-streamable-http-client feature due to missing dependency
- Move From<reqwest::Error> implementation to reqwest module

* feat(rmcp): enhance transport features by decoupling reqwest

- Added reqwest features for reqwest-based implementations.
- Updated documentation
- Modified error handling in SSE transport to use `String` for content type.
- Updated examples to include new features

* feat(rmcp): enhance transport features by decoupling reqwest

- Added reqwest features for reqwest-based implementations
- Updated documentation
- Modified error handling in SSE transport to use `String`
- Updated examples to include new features

examples/rig-integration/Cargo.toml

@@ -17,10 +17,9 @@ rig-core = "0.15.1"
 tokio = { version = "1", features = ["full"] }
 rmcp = { workspace = true, features = [
     "client",
-    "reqwest",
     "transport-child-process",
-    "transport-sse-client",
-    "transport-streamable-http-client"
+    "transport-sse-client-reqwest",
+    "transport-streamable-http-client-reqwest"
 ] }
 anyhow = "1.0"
 serde_json = "1"