feat(ffi): add config server client bindings (#2320)

Add config server client support for the C FFI and Android JNI bindings.

Reuse the existing easytier::web_client::run_web_client path and 
NetworkInstanceManager; OHOS is unchanged.

Report successful remote config apply/delete operations through a 
callback, with one JSON event per affected instance.

Keep the config server client and FFI data plane mutually exclusive: once 
either side is in use, the other side returns an error instead of sharing
lifecycle state.

easytier-contrib/easytier-ffi/Cargo.toml

@@ -4,10 +4,11 @@ version = "0.1.0"
 edition.workspace = true
 
 [lib]
-crate-type = ["cdylib"]
+crate-type = ["cdylib", "rlib"]
 
 [features]
-default = ["ffi-dataplane"]
+default = ["c-abi", "ffi-dataplane"]
+c-abi = []
 ffi-dataplane = ["easytier/ffi-dataplane"]
 
 [dependencies]
@@ -15,9 +16,13 @@ easytier = { path = "../../easytier" }
 
 once_cell = "1.18.0"
 dashmap = "6.0"
+tokio = { version = "1", features = ["rt-multi-thread", "io-util", "time", "sync", "macros"] }
+async-trait = "0.1"
+log = "0.4"
+percent-encoding = "2.3"
+url = "2"
 
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1"
 uuid = "1.17.0"
-tokio = { version = "1", features = ["rt-multi-thread", "io-util", "time", "sync", "macros"] }
 tokio-util = "0.7"