feat: ServiceClient trait, HttpTransport, and SunbeamClient factory

Foundation layer for unified service client wrappers:
- AuthMethod enum (None, Bearer, Header, Token)
- ServiceClient trait with service_name(), base_url(), from_parts()
- HttpTransport with json(), json_opt(), send(), bytes() helpers
- SunbeamClient lazy factory with OnceLock-cached per-service clients
- Feature flags for all service modules (identity, gitea, matrix, etc.)

Bump: sunbeam-sdk v0.2.0

sunbeam-sdk/src/lib.rs

@@ -1,6 +1,8 @@
 #[macro_use]
 pub mod error;
 
+pub mod client;
+
 pub mod auth;
 pub mod checks;
 pub mod cluster;
@@ -17,3 +19,21 @@ pub mod secrets;
 pub mod services;
 pub mod update;
 pub mod users;
+
+// Feature-gated service client modules
+#[cfg(feature = "identity")]
+pub mod identity;
+#[cfg(feature = "matrix")]
+pub mod matrix;
+#[cfg(feature = "opensearch")]
+pub mod search;
+#[cfg(feature = "s3")]
+pub mod storage;
+#[cfg(feature = "livekit")]
+pub mod media;
+#[cfg(feature = "monitoring")]
+pub mod monitoring;
+#[cfg(feature = "lasuite")]
+pub mod lasuite;
+#[cfg(feature = "build")]
+pub mod build;